U.S. patent number 6,653,000 [Application Number 09/964,700] was granted by the patent office on 2003-11-25 for magnetic recording medium comprising a specific azo dye.
This patent grant is currently assigned to Fuji Photo Film Co., Ltd.. Invention is credited to Noboru Jinbo, Tomohiro Kodama, Takekatsu Sugiyama, Akihiko Takeda.
United States Patent |
6,653,000 |
Jinbo , et al. |
November 25, 2003 |
Magnetic recording medium comprising a specific azo dye
Abstract
Provided is a magnetic recording medium that is employed to
particular advantage as an external recording medium for the
recording of digital data, having a layer in which carbon black is
well dispersed, and in particular, a backcoat layer. A magnetic
recording medium comprising a layer comprising a granular substance
and a binder, wherein said layer comprises the compound denoted by
general formula (I) below. (in general formula (I), A denotes a
component forming an azo dye with X--Y; X denotes a group selected
from among the divalent connecting groups denoted by the structural
formulas below: ##STR1## (in general formula (II), Z denotes a
lower alkylene group; --NR.sub.2 denotes a lower alkylamino group
or a nitrogen-comprising five-membered or six-membered heterocyclic
ring; and a denotes 1 or 2)).
Inventors: |
Jinbo; Noboru (Odawara,
JP), Takeda; Akihiko (Fujinomiya, JP),
Sugiyama; Takekatsu (Fujinomiya, JP), Kodama;
Tomohiro (Fujinomiya, JP) |
Assignee: |
Fuji Photo Film Co., Ltd.
(Kanagawa, JP)
|
Family
ID: |
18781483 |
Appl.
No.: |
09/964,700 |
Filed: |
September 28, 2001 |
Foreign Application Priority Data
|
|
|
|
|
Sep 29, 2000 [JP] |
|
|
2000-299711 |
|
Current U.S.
Class: |
428/845.2;
G9B/5.243; G9B/5.285; 428/323; 428/329; 428/838; 428/845;
428/839.5; 428/843 |
Current CPC
Class: |
G11B
5/70 (20130101); G11B 5/7358 (20190501); G11B
5/7356 (20190501); Y10T 428/25 (20150115); Y10T
428/257 (20150115) |
Current International
Class: |
G11B
5/70 (20060101); G11B 5/62 (20060101); G11B
5/735 (20060101); B32B 005/16 () |
Field of
Search: |
;428/694B,694BB,694BR,694BN,323,329 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Le; H. Thi
Attorney, Agent or Firm: Sughrue Mion, PLLC
Claims
What is claimed is:
1. A magnetic recording medium comprising a layer comprising a
granular substance and binder, wherein said layer comprises a
compound denoted by general formula I below:
2. The magnetic recording medium according to claim 1, wherein said
granular substance is carbon black and said layer comprising said
granular substance and binder is a backcoating layer provided on a
nonmagnetic support on the reverse side from the surface on which a
magnetic layer is provided.
3. The magnetic recording medium according to claim 2, wherein said
magnetic recording medium comprises a nonmagnetic layer and the
magnetic layer in this order on the nonmagnetic support.
4. The magnetic recording medium according to claim 3, wherein the
compound denoted by general formula (I) is further comprised in the
magnetic layer and/or nonmagnetic layer.
5. The magnetic recording medium according to claim 2, wherein an
essentially nonmagnetic lower layer and a magnetic layer comprised
of a ferromagnetic micropowder dispersed in binder are provided in
this order on the nonmagnetic support, the coercivity of the
magnetic layer is equal to or higher than 1.43.times.10.sup.5 A/m
(1,800 Oe), the product of the saturation magnetic flux density and
the magnetic layer thickness of the magnetic layer ranges from 5 to
300 (mT .mu.m), and the surface roughness of the magnetic layer
ranges from 1.0 to 3.0 nm as the center surface average surface
roughness as measured by an optical interference roughness
meter.
6. The magnetic recording medium according to claim 5, wherein the
compound denoted by general formula (I) is further comprised in the
magnetic layer and/or essentially nonmagnetic layer.
7. The magnetic recording medium according to claim 1, wherein said
magnetic recording medium has a nonmagnetic support, on one side of
which is provided a magnetic layer, and on the other side of which
is provided a backcoating layer, wherein the backcoating layer
comprises microgranular carbon black with a mean primary particle
diameter ranging from 5 to 30 nm, binder, and the compound denoted
by the general formula (I).
8. The magnetic recording medium according to claim 7, wherein said
microgranular carbon black has a mean primary particle diameter
ranging from 5 to 30 nm, a specific surface area ranging from 60 to
800 m.sup.2 /g, a DBP oil absorption capacity ranging from 50 to
130 mL/100 g, pH ranging from 2 to 11, and a volatile content equal
to or less than 15 weight percent.
9. The magnetic recording medium according to claim 1, wherein said
magnetic recording medium has a nonmagnetic support, on one side of
which is provided a magnetic layer, and on the other side of which
is provided a backcoating layer, wherein the backcoating layer
comprises microgranular carbon black with a mean primary particle
diameter ranging from 5 to 30 nm, coarse granular carbon black with
a mean primary particle diameter ranging from 40 to 360 nm, binder,
and the compound denoted by the general formula (I).
10. The magnetic recording medium according to claim 9, wherein
said microgranular carbon black has a mean primary particle
diameter ranging from 5 to 30 nm, a specific surface area ranging
from 60 to 800 m.sup.2 /g, a DBP oil absorption capacity ranging
from 50 to 130 mL/100 g, pH ranging from 2 to 11, and a volatile
content equal to or less than 15 weight percent.
11. The magnetic recording medium according to claim 9, wherein
said coarse granular carbon black has a mean primary particle
diameter ranging from 40 to 360 nm, a specific surface area ranging
from 5 to 70 m.sup.2 /g, a DBP oil absorption capacity ranging from
20 to 100 mL/100 g, and pH ranging from 5 to 11.
12. The magnetic recording medium according to claim 1, wherein
said magnetic recording medium has a nonmagnetic support, on one
side of which is provided a magnetic layer, and on the other side
of which is provided a backcoating layer, wherein the backcoating
layer is formed by dispersing a mixture of microgranular carbon
black with a mean primary particle diameter ranging from 5 to 30
nm, binder, the compound denoted by the general formula (I), and
nitrocellulose that has been wetted with any compound from among
the group consisting of aromatic hydrocarbon compounds, ketone
compounds, and ether compounds to form a carbon black coating
material, adding a curing agent thereto, and coating the
mixture.
13. The magnetic recording medium according to claim 12, wherein
said microgranular carbon black has a mean primary particle
diameter ranging from 5 to 30 nm, a specific surface area ranging
from 60 to 800 m.sup.2 /g, a DBP oil absorption capacity ranging
from 50 to 130 mL/100 g, pH ranging from 2 to 11, and a volatile
content equal to or less than 15 weight percent.
14. The magnetic recording medium according to claim 1, wherein
said magnetic recording medium has a nonmagnetic support, on one
side of which is provided a magnetic layer, and on the other side
of which is provided a backcoating layer, wherein the backcoating
layer comprises microgranular carbon black with a mean primary
particle diameter ranging from 5 to 30 nm and an inorganic powder
with a mean particle diameter ranging from 10 to 250 nm and a Mohs'
hardness ranging from 5 to 9, and the surface roughness Ra of the
backcoating layer ranges from 2.0 to 15 nm.
15. The magnetic recording medium according to claim 14, wherein
said inorganic powder is .alpha.-iron oxide or .alpha.-alumina.
16. The magnetic recording medium according to claim 14, wherein
said microgranular carbon black has a mean primary particle
diameter ranging from 5 to 30 nm, a specific surface area ranging
from 60 to 800 m.sup.2 /g, a DBP oil absorption capacity ranging
from 50 to 130 mL/100 g, pH ranging from 2 to 11, and a volatile
content equal to or less than 15 weight percent.
17. The magnetic recording medium according to claim 1, wherein
said magnetic recording medium has a nonmagnetic support, on one
side of which is provided a magnetic layer and on the other side of
which is provided a backcoating layer, wherein the thickness of the
backcoating layer ranges from 0.2 to 8.0 .mu.m, the total thickness
of the medium ranges from 3 to 10 .mu.m, and the surface roughness
Ra of the backcoating layer ranges from 2.0 to 15 nm.
Description
FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium
employed with particular advantage as an external magnetic
recording medium for recording digital data.
RELATED ART
Magnetic recording media are widely employed in recording tapes,
video tapes, computer tapes, disks, and the like. Magnetic
recording media are becoming denser and the recording wavelengths
are becoming shorter each year. There is also an examination
underway as to whether to change the recording method from analog
to digital.
Magnetic recording media employing a thin metal layer as the
recording layer are being examined in response to the demand for
higher density. However, with regard to such practical
reliabilities as production properties and corrosiveness, so-called
particulate magnetic recording media in which a ferromagnetic
powder is dispersed in binder and coated on a support are superior.
However, particulate media have poorer electromagnetic
characteristics than thin metal films due to low fill rates of
magnetic material.
Particulate magnetic recording media are widely employed in which a
magnetic layer comprised of a ferromagnetic iron oxide, Co-modified
ferromagnetic iron oxide, CrO.sub.2, ferromagnetic alloy powder, or
the likes dispersed in binders is coated on a support.
Numerous methods of improving the electromagnetic characteristics
of particulate magnetic recording media have been proposed, such as
improving the magnetic characteristics of the ferromagnetic powder
and smoothing the surface. However, these are not adequate for
achieving higher density. Further, in recent years, there has been
a tendency to shorten the recording wavelength in combination with
higher densities; the problems of thickness loss during
reproduction and self-demagnetization loss, where output drops
during recording as the magnetic layer becomes thinner, have become
significant. Accordingly, ultrathin-layer particulate magnetic
recording media have been proposed.
For example, Japanese Unexamined Patent Publication (KOKAI) Heisei
No. 6-2153650 discloses relatively reducing the total thickness of
the magnetic tape and the thickness of the backcoating layer. As
specific examples of the magnetic tape described in this
publication, in one embodiment the total thickness of the magnetic
tape is 10 .mu.m and the thickness of the backcoating layer is 0.5
.mu.m, and in another the total thickness is 9.5 .mu.m and the
thickness of the backcoating layer is 0.5 .mu.m. To impart
antistatic and running stability properties to the backcoating
layer in such embodiments, comparatively microgranular carbon black
alone is employed in the former embodiment, and two types of carbon
black, one being comparatively microgranular carbon black and the
other being comparatively coarse carbon black, are employed in the
latter embodiment.
Additionally, a magnetic tape has been proposed in which
microgranular carbon black of a mean particle diameter ranging from
10 to 80 nm, coarse granular carbon black with a mean particle
diameter ranging from 150 to 500 nm, and microgranular calcium
carbonate of a mean particle diameter ranging from 10 to 45 nm are
incorporated into the backcoating layer to achieve high surface
smoothness in the backcoating layer, reduce the coefficient of
friction with guide pins, and achieve good running stability
(Japanese Unexamined Patent Publication (KOKAI) Heisei No. 2-7223).
The further introduction into the backcoating layer of inorganic
powder (for example, .alpha.-iron oxide) is also described.
Increasing the bulk recording density of particulate magnetic
recording media is nearly equivalent to employing thin layers to
reduce the total thickness of the tape. In particulate recording
media, it is also necessary to employ thin layers in the
backcoating layer; the thinner it becomes, the greater the
requirements for improved dispersion and improved coating strength.
Further, proper surface smoothness with repeat running stability is
required. However, dispersion of the granular substances such as
carbon black that are employed as the principal starting material
of the backcoating layer is extremely difficult. Even when kneading
treatment, sandmill dispersion, and the like are combined, there
are limits to the improvement in dispersion, coating strength, and
coating smoothness that can be achieved. In particulate magnetic
recording media, there is also the problem of poor dispersion of
granular substances in layers other than the backcoating layer.
SUMMARY OF THE INVENTION
Accordingly, the object of the present invention is to provide a
magnetic recording medium that can be employed with particular
advantage as an external recording medium to record digital data,
having layers, particularly a backcoating layer, in which granular
substances such as carbon black are well dispersed. In particular,
the object of the present invention is to provide a magnetic
recording medium, in which, even in a backcoating layer that has
been reduced in thickness, the granular substances such as carbon
black contained therein are well dispersed, and good strength and
smoothness are afforded.
DETAILED EXPLANATION OF THE INVENTION
The present inventors expended considerable research effort to
achieve a magnetic recording medium having a layer comprising
binder and granular substances and with well-dispersed granular
substances such as carbon black and with good strength and surface
smoothness. As a result, they discovered that incorporating the
compound denoted by general formula (I) below improved the
dispersion of granular substances within the layer; the present
invention was devised on that basis.
That is, the object of the present invention is achieved by a
magnetic recording medium having a layer comprising a granular
substance and binder, characterized in that said layer comprises
the compound denoted by general formula (I) below:
(in general formula (I), A denotes a compound capable of forming an
azo dye with X--Y, X denotes a group selected from the divalent
connecting groups denoted by the structural formulas given below
##STR2##
and Y denotes a group denoted by general formula (II) below
##STR3## (in general formula (II), Z denotes a lower alkylene
group, --NR.sub.2 denotes a lower alkylamino group or a
nitrogen-comprising five-membered or six-membered saturated
heterocyclic ring, and a denotes 1 or 2).
Preferred Aspects of the magnetic recording medium of the present
invention are given below: 1. A particulate magnetic recording
medium having a nonmagnetic support, on one side of which is
provided a magnetic layer, and on the other side of which is
provided a backcoating layer, with the backcoating layer comprising
microgranular carbon black with a mean primary particle diameter
ranging from 5 to 30 nm, binder, and the compound denoted by
general formula (I) above; 2. A particulate magnetic recording
medium having a nonmagnetic support, on one side of which is
provided a magnetic layer, and on the other side of which is
provided a backcoating layer, with the backcoating layer comprising
microgranular carbon black with a mean primary particle diameter
ranging from 5 to 30 nm, coarse granular carbon black with a mean
primary particle diameter ranging from 40 to 360 nm, binder, and
the compound denoted by general formula (I) above; 3. A particulate
magnetic recording medium having a nonmagnetic support, on one side
of which is provided a magnetic layer, and on the other side of
which is provided a backcoating layer, where the backcoating layer
is formed by dispersing a mixture of microgranular carbon black
with a mean primary particle diameter ranging from 5 to 30 nm,
binder, the compound denoted by general formula (I) above, and
nitrocellulose that has been wetted with any compound from among
the group consisting of aromatic hydrocarbon compounds, ketone
compounds, and ether compounds to form a carbon black coating
material, adding a curing agent thereto, and coating the mixture.
4. A particulate magnetic recording medium having a nonmagnetic
support, on one side of which is provided a magnetic layer, and on
the other side of which is provided a backcoating layer,
characterized in that the backcoating layer comprises microgranular
carbon black with a mean primary particle diameter ranging from 5
to 30 nm and an inorganic powder with a mean particle diameter
ranging from 10 to 250 nm and a Mohs' hardness ranging from 5 to 9,
and in that the surface roughness Ra thereof ranges from 2.0 to 15
nm. 5. A magnetic recording medium in which the inorganic powder
with a Mohs' hardness ranging from 5 to 9 of Aspect 4 is
.alpha.-iron oxide or .alpha.-alumina. 6. A magnetic recording tape
having a nonmagnetic support, on one side of which is provided a
magnetic layer and on the other side of which is provided a
backcoating layer, where the thickness of the backcoating layer
ranges from 0.2 to 8.0 .mu.m, the total thickness of the tape
ranges from 3 to 10 .mu.m, and the surface roughness Ra of the
backcoating layer ranges from 2.0 to 15 nm. 7. A magnetic recording
medium having a nonmagnetic support, on one side of which is
provided a magnetic layer and on the other side of which is
provided a backcoating layer, where an essentially nonmagnetic
lower layer and a magnetic layer comprised of a ferromagnetic
micropowder dispersed in binder are provided in this order on the
nonmagnetic support, the coercivity of the magnetic layer is equal
to or higher than 1.43.times.10.sup.5 A/m (1,800 Oe), the product
of the saturation magnetic flux density and the magnetic layer
thickness of the magnetic layer ranges from 5 to 300
(mT.multidot..mu.m), and the surface roughness of the magnetic
layer ranges from 1.0 to 3.0 nm as the center surface average
surface roughness as measured by an optical interference roughness
meter. 8. A magnetic recording medium where, in Aspects 1 to 4
above, the mean primary particle diameter of the microgranular
carbon black ranges from 5 to 30 nm, the specific surface area
ranges from 60 to 800 m.sup.2 /g, the DBP oil absorption capacity
ranges from 50 to 130 mL/100 g, the pH ranges from 2 to 11, and the
volatile content is equal to or less than 15 weight percent. 9. A
magnetic recording medium wherein, in Aspect 2 above, the mean
primary particle diameter of the coarse granular carbon black
ranges from 40 to 360 nm, the specific surface area ranges from 5
to 70 m.sup.2 /g, the DBP oil absorption capacity ranges from 20 to
100 mL/100 g, and the pH ranges from 5 to 11.
The magnetic recording medium of the present invention is described
in greater detail below.
The magnetic recording medium of the present invention comprises
the compound denoted by general formula (I) below.
In general formula (I) above, A denotes a compound capable of
forming an azo dye with X--Y. Said A may be any compound capable of
coupling with a diazonium compound to form an azo dye.
Specific examples of said A are given below, but the present
invention is not limited in any way thereto. ##STR4##
In general formula (I) above, X denotes a group selected from among
divalent connecting groups denoted by the following structural
formulas: ##STR5##
In general formula (I) above, Y denotes a group denoted by general
formula (II): ##STR6##
In general formula (II), Z denotes a lower alkylene group. Z may be
represented by --(CH.sub.2).sub.b --, wherein b denotes an integer
ranging from 1 to 5, preferably 2 or 3.
In general formula (II), --NR.sub.2 denotes a lower alkylamino
group or a nitrogen-comprising five-membered or six-membered
heterocyclic ring. When --NR.sub.2 denotes a lower alkylamino
group, it may be represented by --N(C.sub.n H.sub.2n+1).sub.2,
where n denotes an integer ranging from 1 to 4, preferably 1 or 2.
Additionally, when --NR.sub.2 denotes a nitrogen-comprising
five-membered or six-membered heterocyclic ring, the heterocyclic
ring is preferably represented by one of the following structural
formulas: ##STR7##
In general formula (II) above, Z and --NR.sub.2 are optionally
substituted with a lower alkyl group or alkoxyl group.
In general formula (II) above, a denotes 1 or 2, preferably 2.
Specific examples of the compound denoted by general formula (I)
above are given below, but the present invention is not limited in
any way thereto. ##STR8## ##STR9## ##STR10## ##STR11##
Synthesis examples of the compound denoted by general formula (I)
above are given below.
SYNTHESIS EXAMPLE 1
Synthesis of Compound Example 2 (1) Fifty parts of dimethyl
5-nitroisophthalate and 130 parts of N,N-diethyl-1,3-propanediamine
were reacted for about four hours at 80 to 100.degree. C. under a
slight vacuum. After confirming the disappearance of the starting
material dimethyl 5-nitroisophthalate and monoamide compounds, the
excess N,N-diethyl-1,3-propanediamine was removed under vacuum,
yielding 92 parts of 5-nitroisophthalic acid
bis-3-diethylaminopropylamide. (2) 92 parts of the
5-nitroisophthalic acid bis-3-diethylaminopropylamide obtained, 112
parts of reduced iron, and 12 parts of ammonium chloride were
refluxed in 200 parts of isopropanol and 35 parts of water and
reduced, yielding 86 parts of 5-aminoisophthalic acid
bis-3-diethylaminopropylamide. (3) 18.5 parts of the
5-nitroisophthalic acid bis-3-diethylaminopropylamide obtained and
5.1 parts of triethylamine were dissolved in 60 parts of DMF and
cooled with ice.
To this was added a solution comprising 9.3 parts of 4-nitrobenzoyl
chloride in 60 parts of acetone, and amidation was conducted.
Following the reaction, 800 parts of water were added, the crystals
were recovered by filtration, and the crystals were recrystallized
from ethyl acetate, yielding 14 parts of
4-nitrobenzoyl-4-[3,5-bis(3-diethylaminopropylcarbamoyl)]phenylamide.
(4) The compound obtained was reduced in the same manner as in (2)
above, yielding 13.2 parts of aniline derivative. (5) 13.2 parts of
the aniline derivative obtained were added to 120 parts of
methanol. With ice cooling, 18 parts of hydrochloric acid were then
added. The mixed solution was then further cooled to -15.degree.
C.
To this was added dropwise an aqueous solution (20 parts water) of
1.8 parts of NaNO.sub.2 to conduct diazotization (preparation of a
diazo solution). A coupling component solution comprising 5.9 parts
of 5-acetoacetylaminobenzimidazolone, 260 parts of methanol, 530
parts of water, and 10.8 parts of NaCO.sub.3 was separately
prepared and cooled to below 10.degree. C. The diazo solution
obtained above was then added dropwise without exceeding a
temperature of 10.degree. C. and the two were reacted. K.sub.2
CO.sub.3 was added to render the system basic. The precipitating
yellow product was recovered by filtration and recrystallized from
DMF and acetonitrile, yielding 19 parts of above-recorded compound
2. The maximum absorption wavelength of the compound obtained was
.lambda. max 391 nm (in CHCl.sub.3).
SYNTHESIS EXAMPLE 2
Synthesis of Compound 11 (1) A diazo solution comprising 180 parts
of methanol, 31 parts of hydrochloric acid, 3.1 parts of
NaNO.sub.2, and 30 parts of water was prepared with 22.9 parts of
aniline derivative in the same manner as in (5) of Synthesis
Example 1. (2) A coupling component solution comprising 5.6 parts
of barbituric acid, 600 parts of methanol, 1,100 parts of water,
and 19 parts of Na.sub.2 CO.sub.3 was prepared. To this was added
the diazo solution obtained in (1) and the two were reacted.
Following the reaction, K.sub.2 CO.sub.3 was added to render the
system basic. The precipitating crystals were recovered by
filtration and recrystallized from DMF and acetonitrile, yielding
16.3 parts of above-recorded compound 11. The maximum absorption
wavelength of the compound obtained was .lambda. max 378 nm (in
CHCl.sub.3).
The magnetic recording medium of the present invention has a layer
comprising a granular substance. Examples of granular substances
are metal oxides such as hematite, magnetite, maghemite, bertholide
compounds, barium ferrite compounds, goethite, TiO.sub.2, alumina,
and boehmite; carbon black; metal and ferromagnetic metal powders;
and Fe and FeCo alloys. The incorporation of carbon black is
preferred to achieve an antistatic effect and running
stability.
The above-described layer comprising a granular substance can be a
magnetic layer, nonmagnetic layer, and/or backcoating layer;
preferably, it is a backcoating layer provided on the reverse side
of the nonmagnetic support from the surface on which the magnetic
layer is provided. Providing a backcoating layer comprising a
granular substance such as carbon black affords better running
stability.
Preferably, the compound denoted above by general formula (I) above
of the present invention is added in a quantity ranging from 0.1 to
50 parts by weight with respect to 100 parts by weight of the
granular substance. When the amount added is within the stated
range, good dispersion of the granular substance is achieved. The
solvent employed in the step of manufacturing the granular
dispersion layer of the present invention is not specifically
limited, it being permissible to employ water, an organic solvent,
or a mixed solution thereof.
It is possible to the granular substance is dispersed in a solvent
containing the compound denoted by general formula (I) above, a
granular substance dispersion coating material is prepared, and
this coating material is coated, heated, and cured by the usual
coating methods to provide a layer in which the granular substance
is well dispersed.
To achieve a good dispersion, the granular substance is admixed
during kneading of the compound denoted by general formula (I)
above and the binder in an open kneader or the like, or during
dispersion thereof using a roll mill or sand mill. Hot mixing of
the compound of general formula (I) above and the binder in a melt
state prior to dispersion is desirable not only to improve affinity
between the basic groups of the compound of general formula (I)
above and the binder, but also to improve quality and production
properties because admixing of the granular substance is then
uniform and rapid.
A binder such as nitrocellulose, dispersants, curing agents,
lubricants, and the like may be incorporated into the
above-described granular substance dispersion coating material.
Specifically, the incorporation of nitrocellulose that has been
wetted with any of the compounds in the group consisting of
aromatic hydrocarbon compounds, ketone compounds, and ether
compounds can yield better dispersion (above-described preferred
Aspect 3).
When carbon black is incorporated into the backcoating layer,
microgranular carbon black having a mean particle diameter ranging
from 5 to 30 nm, for example, is preferred (above-described
preferred Aspect 1). The type and manufacturing history of the
carbon black are not specifically limited. Commercial oil furnace
black, gas furnace black, channel black, and various other
microgranular carbon blacks may be employed. Further, carbon black
that has been subjected to a commonly conducted ozone treatment,
plasma treatment, or liquid phase oxide treatment may also be
employed.
The microgranular carbon black of particular preference has a mean
particle diameter ranging from 5 to 30 nm, a specific surface area
ranging from 60 to 800 m.sup.2 /g, a DBP oil absorption capacity
ranging from 50 to 130 mL/100 g, a pH ranging from 2 to 11, and a
volatile content equal to or less than 15 weight percent
(above-described preferred Aspect 8).
Generally, the addition of microgranular carbon black makes it
possible to impart low surface resistivity and low optical
transmittance to the backcoating layer. Since the optical
transmittance of the tape is often exploited by the magnetic
recording device for use as an operation signal, in such a case,
the addition of microgranular carbon black can be particularly
effective. Generally, microgranular carbon black has good liquid
lubricant retention ability and contributes to a reduction in the
coefficient of friction. Additionally, coarse granular carbon black
having a particle size ranging from 40 to 360 nm functions as a
solid lubricant, forms minute protrusions on the surface of the
backcoating layer, reduces the contact surface area, and
contributes to a reduction in the coefficient of friction. However,
in severe running systems, coarse granular carbon black has a
drawback in that it tends to fall out of the backcoating layer due
to rubbing of the tape, thus increasing the error rate.
Accordingly, two types of carbon black of differing mean particle
size are preferably combined for use as the carbon black that is
added to the backcoating layer. In that case, microgranular carbon
black having a mean particle size ranging from 5 to 30 nm and
coarse granular carbon black having a mean particle size ranging
from 40 to 360 nm are preferably combined for use (preferred Aspect
2). The mean particle diameter of the coarse granular carbon black
is suited to range from 40 to 360 nm, preferably from 200 to 350
nm. More preferably, the mean particle diameter of the coarse
granular carbon black above falls with the range of from 40 to 360
nm, the specific surface area thereof falls within the range of
from 5 to 70 m.sup.2 /g, the DBP oil absorption capacity thereof
falls within the range of from 20 to 100 mL/100 g, and the pH
thereof falls within the range of from 5 to 11 (above-described
preferred implementation mode 9).
When microgranular carbon black and coarse granular carbon black
are employed in combination, the mass ratio thereof
(microparticle/coarse particle) preferably ranges from 99/1 to
70/30, particularly from 99/1 to 80/20.
Specific products of microgranular carbon black that may be
employed in the backcoating layer are given below. Numbers in
parentheses denote mean particle diameters: RAVEN 2500 ULTRA (13
nm), RAVEN 5000 (8 nm), RAVEN 5000 ULTRA II (8 nm), RAVEN 5000
ULTRA III (8 nm), RAVEN 3500 (13 nm), RAVEN 5250 (16 nm), RAVEN
5750 (12 nm), RAVEN 1250 (20 nm), RAVEN 1200 (20 nm), RAVEN 2000
(18 nm), RAVEN 1500 (17 nm), RAVEN 1100 Ultra (27 nm), RAVEN 1170
(21 nm), RAVEN 1080 Ultra (28 nm), RAVEN 1060 Ultra (30 nm), RAVEN
1040 (24 nm), RAVEN 1020 (24 nm), RAVEN 890H (28 nm), Conductex 975
Ultra (21 nm), RAVEN 880 Ultra (30 nm), RAVEN 780 Ultra (29 nm),
RAVEN 760 Ultra (30 nm), Conductex SC Ultra (20 nm), RAVEN C Ultra
(20 nm) (the above products are manufactured by Columbia Carbon
Co., Ltd.); MONARCH 800 (17 nm), BLACK PEARLS 800 (17 nm), MONARCH
880 (16 nm), BLACK PEARLS 880 (16 nm), MONARCH 900 (15 nm), BLACK
PEARLS 900 (15 nm), MONARCH 1000 (16 nm), BLACK PEARLS 1000 (16
nm), MONARCH 1100 (14 nm), BLACK PEARLS 1100 (14 nm), MONARCH 1300
(13 nm), BLACK PEARLS 1300 (13 nm), MONARCH 1400 (13 nm), BLACK
PEARLS 1400 (13 nm), VULCAN P (20 nm), BLACK PEARLS 480 (29 nm),
MONARCH 460 (28 nm), BLACK PEARLS 460 (28 nm), BLACK PEARLS 430 (27
nm), REGAL 330R (25 nm), REGAL 330 (25 nm), REGAL 415R (25 nm),
REGAL 415 (25 nm), VULCAN 9A32 (19 nm), REGAL 400R (25 nm), REGAL
400 (25 nm), REGAL 660R (24 nm), REGAL 660 (24 nm), MOGUL-L (24
nm), BLACK PEARLS-L (24 nm), REGAL 500R (25 nm) (the above products
are manufactured by Cabot Corporation); PRINTEX 90 (14 nm), PRINTEX
95 (15 nm), PRINTEX 85 (16 nm), PRINTEX 75 (17 nm), Printex 55 (25
nm), Printex 45 (26 nm), Printex 40 (26 nm), Printex P (20 nm),
Printex 60 (21 nm), Printex L6 (18 nm), Printex L (23 nm), Printex
300 (27 nm), Printex 30 (27 nm), Printex 3 (27 nm), Special Black
550 (25 nm) (the above products are manufactured by Degusa Co.);
#3950, #950 (16 nm), #650B (22 nm), #2600 (13 nm), #2400 (15 nm),
#2350 (15 nm), #2300 (15 nm), #2200 (18 nm), #1000 (18 nm), MA-600
(20 nm), #4000 (20 nm), #9180 (13 nm), #2700B (13 nm), #2650B (13)
nm), #2450B (15 nm), #2400B (15 nm), #2200B (18 nm), #990 (16 nm),
#980 (16 nm), #970 (16 nm), #960 (16 nm), #900 (16 nm), MCF88 (18
nm), #850 (17 nm), #750B (22 nm), #52 (27 nm), #50 (28 nm), #47 (23
nm), #45 (24 nm), #45L (24 nm), #44 (24 nm), #40 (24 nm), #33 (30
nm), #32 (30 nm), #30 (30 nm), MA77 (23 nm), MA7 (24 nm), MA8 (24
nm), MA11 (29 nm), MA100 (24 nm), MA100R (24 nm), MA100S (24 nm),
MA230 (30 nm), MA200RB (30 nm) (manufactured by Mitsubishi Chemical
Corporation).
Examples of specific products of coarse granular carbon black are
Thermal Black (270 nm) (manufactured by Cancarb Limited.), RAVEN
MTP (275 nm), Sevacarb MT-CI (350 nm), RAVEN 430 Ultra (82 nm),
RAVEN 520 (60 nm), RAVEN 500 (53 nm), RAVEN 460 (67 nm), RAVEN 450
(75 nm), RAVEN 420 (86 nm), RAVEN 410 (101 nm), RAVEN H2O (55 nm)
(manufactured by Columbia Carbon Co., Ltd.); BLACK PEARLS 130 (75
nm), REGAL 350R (48 nm), REGAL 350 (48 nm) (manufactured by Cabot
Corporation); #25 (47 nm), #10 (75 nm), #5 (76 nm), CF9 (40 nm),
#95 (40 nm), #260 (40 nm), #4010B (75 nm), MA14 (40 nm), and MA 220
(55 nm) (manufactured by Mitsubishi Chemical Corporation).
Further, inorganic powders with a Mohs' hardness of from 5 to 9 may
be employed in combination with the above-listed microgranular
carbon blacks (above-described preferred implementation mode 4).
Two or more types of inorganic powder may be combined for use.
Examples of preferred inorganic powders are .alpha.-iron oxide and
.alpha.-alumina (above-described preferred implementation mode 5).
The mass ratio in this case (microgranular carbon black/inorganic
powder) preferably ranges from 99.5/0.5 to 70/30, particularly from
99/1 to 80/20.
Further, in addition to microgranular carbon black and inorganic
powder, coarse granular carbon black may also be employed in
combination. In that case, the mass ratio of the microgranular
carbon black to the coarse granular carbon black preferably falls
within the above-stated range, and the mass ratio (total carbon
black/inorganic powder) preferably ranges from 99/1 to 70/30.
The surface roughness of the backcoating layer, in the form of the
center surface average surface roughness Ra measured by optical
interference roughness meter, preferably ranges from 2 to 15 nm,
more preferably from 2 to 10 nm (above-described preferred Aspect
4).
When the magnetic tape is wound up, the surface of the backcoating
layer is transferred to the surface of the magnetic layer, with the
surface roughness affecting reproduction output and affecting the
coefficient of friction to the guide poles. Thus, the surface
roughness is desirably adjusted to within the above-stated range.
Surface roughness Ra is normally adjusted by adjusting the
material, surface properties, pressure, and the like of the
calender rolls employed in the step of surface processing by
calender following coating formation of the backcoating layer. In
the present invention, the backcoating layer preferably ranges from
0.2 to 0.8 .mu.m, more preferably from 0.2 to 0.7 .mu.m, in
thickness. In this case, the overall thickness of the tape
preferably ranges from 3 to 10 .mu.m, more preferably from 3 to 9.5
.mu.m (preferred Aspect 6).
The magnetic layer employed in a magnetic recording medium having
such a backcoating layer preferably has a surface recording density
ranging from 0.3 to 3 Gbit/inch.sup.2, more preferably from 0.5 to
3 Gbit/inch.sup.2. Such a high surface recording density can also
be achieved in magnetic recording media having single-layer
magnetic layers, but are also effectively achieved in a magnetic
recording medium having a configuration in which a magnetic layer
is provided on a nonmagnetic lower layer. Although the magnetic
layer may be in the form of a particulate medium comprising a
ferromagnetic powder and binder, or in the form of a thin metal
layer formed by a vacuum film forming method such as vapor
deposition, a particulate medium is preferred from the viewpoint of
production properties and the like, particularly when a lower layer
is provided.
In magnetic recording media possessing both good durability and
high density characteristics in the form of a surface recording
density ranging from 0.3 to 3 Gbit/inch.sup.2, or from 0.5 to 3
Gbit/inch.sup.2, points such as those below may be organically
linked: (1) high Hc and ultrasmoothness; (2) durability ensured by
composite lubricating agents and improvements in high durability
binders and ferromagnetic powders; (3) ultrathin magnetic layer and
reduced variation in the interface with the lower layer; (4) high
packing of ferromagnetic powder; (5) powders (ferromagnetic powder,
nonmagnetic powder) with ultrafine microparticles; (6) head touch
stabilization; (7) dimensional stabilization and servo tracking;
(8) improvement in the thermal shrinkage rate of the magnetic layer
and support; and (9) the effect of lubricants at high and low
temperatures.
In the magnetic recording medium obtained based on the present
invention, preferred is that the ultrathin magnetic layer comprises
ultrafine granular magnetic powder with high output, good
dispersion, and good durability, the lower layer comprises
spherical, acicular, or similar inorganic powder, the use of a thin
magnetic layer is desirable in that it reduces the effect of
self-demagnetization in the magnetic layer, significantly increases
output in the high frequency range, and improves overwrite
characteristics. Improvement in the magnetic head further enhances
the effect of the ultrathin magnetic layer when combined with a
narrow-gap head and improves digital recording characteristics. The
use of an MR element as the reproduction head is desirable in
systems employing giant magnetic resistive elements.
To bring the magnetic layer thickness into conformity with
high-density magnetic recording systems and the performance
demanded of magnetic heads, a thin layer of from 0.04 to 0.3 .mu.m
is preferably selected. In an ultrathin magnetic layer that is both
uniform and thin in this manner, the microgranular magnetic powder
and nonmagnetic powder are dispersed to a high degree, thereby a
high level of filling can be achieved. To achieve a maximum degree
of the high-density region suitability, the magnetic powder
employed preferably affords high output, high dispersability, and
high orientation properties. That is, a ferromagnetic metal
micropowder having extremely small particles that is capable of
high output, particularly having a mean major axis length equal to
or less than 0.12 .mu.m, with the crystalline size of the
ferromagnetic metal powder being from 8 to 18 nm, further
incorporating a large quantity of Co and incorporating
antisintering agents in the form of Al and Y compounds can yield
high output and high durability. Since microgranular hexagonal
ferrite has substantial high-density characteristics based on
vertical magnetic anisotropy, its use in the present invention is
desirable. The coercivity (Hc) of the magnetic layer of the
magnetic recording medium of the present invention is preferably
equal to or higher than 143 kA/m, more preferably equal to or
higher than 159 kA/m, and even more preferably from 175 to 400 kA/m
(above-described Aspect 7). The upper limit is not clearly
established, and it is thought that the upper limit may increase
with improvements in recording heads. The saturation magnetic flux
density (Bs) of the magnetic layer preferably ranges from 180 to
650 mT. The product (Bs.multidot..delta.) of the saturation
magnetic flux density (Bs) of the magnetic layer and the thickness
of the magnetic layer (.delta.) desirably falls within the range of
from 5 to 300 (mT.multidot..mu.m) (above-described Aspect 7). The
coercivity, thickness, and Bs.multidot..delta. of the magnetic
layer are desirably optimized for the head employed in the system.
Designing for an optimal value of Bs.multidot..delta. prevents the
MR head from becoming saturated and output from decreasing.
In the magnetic recording medium of the present invention, the
surface roughness of the magnetic layer, in the form of the center
surface average surface roughness Ra as measured by optical
interference roughness meter, preferably ranges from 1.0 to 3.0 nm,
more preferably 2.7 nm or less, and still more preferably 2.5 nm or
less (above-described preferred Aspect 7). At 3.0 nm or less,
spacing loss between the magnetic recording medium and the head
decreases and a high-output, low-noise magnetic recording medium
can be obtained.
Durability is an important element in the magnetic recording
medium. In particular, to achieve a high transfer rate, increasing
the rotational speed of the magnetic head by at least five to ten
times relative to that of a conventional recording system, and
increasing the tape running speed by at least five to ten times in
linear drive systems, are desirable. Ensuring the durability of the
medium when magnetic head/cartridge internal parts and the medium
slide against each other at high speed is an important problem.
Means of increasing the durability of the medium include adjusting
the binder formulation to increase the film strength of the medium
itself and adjusting the lubricant formulation to maintain good
sliding properties with the magnetic head. In preferred media
obtained based on the present invention, a three-dimensional
network binder system suited to ultrathin magnetic layers is
employed to ensure the stability and durability of running during
high-speed rotation. Further, effort is expended on the backcoating
layer to achieve a high transfer rate.
Multiple lubricants producing good effects in use environments of
various temperature and humidity are combined for use, with
individual lubricants performing functions over a wide range of
temperatures (high temperature, room temperature, low temperature)
and humidity (high humidity, low humidity), so that overall, a
stable lubricating effect is maintained.
A two-layer, upper and lower layer structure can be employed. The
lubricant tank effect can be imparted to the lower layer so that a
suitable amount of lubricant is constantly being supplied to the
magnetic layer, thereby increasing the durability of the magnetic
layer. The quantity of lubricant that can be contained in an
ultrathin magnetic layer is limited. Simply thinning the magnetic
layer reduces the absolute quantity of lubricant and leads to
deterioration of running durability; thus, it is difficult to
ensure durability. Imparting different functions to an upper and a
lower layer so that they complement each other makes it possible to
achieve both improved electromagnetic characteristics and
durability. The division of functions is particularly effective in
systems in which a magnetic head and a medium slide past each other
at high speed.
In addition to the function of retaining lubricant, the lower layer
can be imparted with the function of controlling surface
resistivity. Generally, a solid electrically conductive material
such as carbon black is often added to the magnetic layer to
control resistivity. Not only does this limit the filling density
of the magnetic material, but as the magnetic film becomes thinner,
surface roughness is affected. Adding a conductive material to the
lower layer can eliminate these drawbacks. The cushioning effect of
the lower layer imparts good calender molding properties, head
touch, and stable running.
As the capacity and density of magnetic recording increase, the
recording track density increases. In the present invention, by
employing a laser beam processing pattern provided on the magnetic
recording medium surface for optical servo tracking, the
traceability of the magnetic head to recording tracks can be
ensured and the recording track density can be increased. In the
magnetic recording medium obtained based on the present invention,
a support with increased isotropic dimensional stability is
employed to further stabilize traceability. The use of an
ultrasmooth support permits an increase in magnetic layer
smoothness.
With the development of a multimedia society, the need for image
recording is becoming increasingly strong not just in the business
world, but also in the home. The preferred high-capacity magnetic
recording medium obtained based on the present invention has ample
ability to respond to the functional and cost requirements of an
image recording medium, as well as simple data such as text and
numbers. The high-capacity medium obtained based on the present
invention is based on particulate magnetic recording media, with
their proven track records. It affords good long-term reliability
and good cost performance. Only by combining various factors such
as those set forth above through synergistic and organic operation
can the preferred high-capacity magnetic recording medium obtained
based on the present invention be achieved.
In the magnetic recording media of the present invention, the
elements of particulate magnetic recording media will be described
further.
Magnetic Layer
There are no particular restrictions on the magnetic recording
medium other than that there be a structure having a magnetic layer
on at least one surface of a support. Magnetic layers may be
provided on both sides. Preferably, the magnetic layer is provided
on a nonmagnetic lower layer (above-described preferred Aspect 7).
For example, the magnetic layer can be provided by both wet-on-wet
methods (W/W), where the lower layer and the magnetic layer are
simultaneously applied on the support while still wet, or where
following coating of the lower layer, the magnetic layer is
provided while the lower layer is still wet in a sequential wet
coating, and wet-on-dry methods (W/D), where the magnetic layer is
provided after the lower layer has dried. (W/W) is preferred from
the viewpoints of producing the thin magnetic layer and production
yields. In (W/W), since the upper layer and lower layer are
simultaneously formed in a multilayer structure, surface treatment
steps such as calendering may be put to effective use to improve
the surface roughness of even ultrathin magnetic layers.
Ferromagnetic Powder
The ferromagnetic powder employed in the magnetic layer is not
specifically limited. However, ferromagnetic metal powders and
hexagonal ferrite powders are preferred.
The ferromagnetic metal powder is preferably a ferromagnetic metal
powder chiefly comprising .alpha.-iron. In addition to prescribed
atoms, the following atoms can be contained in the ferromagnetic
metal powder: Al, Si, Ca, Mg, P, Ti, Cr, Cu, Y, Sn, Sb, Ba, W, La,
Ce, Pr, Nd, Co, Mn, Zn, Ni, Sr, B, and the like. Particularly, the
incorporation of at least one of the following in addition to
.alpha.-iron is desirable: Al, Ca, Mg, Y, Ba, La, Nd, Sm, Co, and
Ni. Alloying Co with Fe is particularly desirable in that
saturation magnetization increases and demagnetization improves.
The content of Co relative to Fe preferably ranges from 1 to 40
atomic percent, more preferably from 15 to 35 atomic percent, and
still more preferably from 20 to 35 atomic percent. The content of
rare earth elements such as Y preferably ranges from 1.5 to 15
atomic percent, more preferably from 3 to 12 atomic percent, and
still more preferably from 4 to 10 atomic percent. The content of
Al preferably ranges from 1.5 to 12 atomic percent, more preferably
from 3 to 10 atomic percent, and still more preferably from 4 to 9
atomic percent. Rare earth elements such as Y, and Al, function as
antisintering agents; their use in combination yields a high
antisintering effect. These ferromagnetic powders may be pretreated
prior to dispersion with dispersing agents, lubricants,
surfactants, antistatic agents, and the like, described further
below. Specific examples are described in Japanese Examined Patent
Publication (KOKOKU) Showa Nos. 44-14090, 45-18372, 47-22062,
47-22513, 46-28466, 46-38755, 47-4286, 47-12422, 47-17284,
47-18509, 47-18573, 39-10307, and 46-39639; and U.S. Pat. Nos.
3,026,215, 3,031,341, 3,100,194, 3,242,005, and 3,389,014.
The ferromagnetic metal powder may contain a small quantity of
hydroxide or oxide. Ferromagnetic metal powders obtained by known
manufacturing methods may be employed. The following are examples:
methods employing a reducing gas such as hydrogen to reduce hydrous
iron oxide or iron oxide that has been treated to prevent
sintering, yielding Fe or Fe--Co particles or the like; methods of
reduction with compound organic acid salts (chiefly oxalates) and
reducing gases such as hydrogen; methods of thermal decomposition
of metal carbonyl compounds; methods of reduction by addition of a
reducing agent such as sodium boron hydride, hypophosphite, or
hydrazine to an aqueous solution of ferromagnetic metal; and
methods of obtaining micropowder by vaporizing a metal in a
low-pressure inert gas. The ferromagnetic metal powders obtained in
this manner are subjected to any of the known gradual oxidation
treatments. The method in which hydrous iron oxide or iron oxide is
reduced with a reducing gas such as hydrogen and partial pressure
of oxygen-containing gas and inert gas, temperature and time are
controlled to form an oxide film on the surface is preferred
because of little demagnetization.
The specific surface area (referred to hereinafter as "S.sub.BET ")
as measured by the BET method of the ferromagnetic powder ranges
from 40 to 80 m.sup.2 /g, preferably from 45 to 70 m.sup.2 /g. At
less than 40 m.sup.2 /g, noise sometimes increases, and at greater
than 80 m.sup.2 /g, good surface properties become difficult to
achieve. The crystalline size of the ferromagnetic metal powder of
the magnetic layer of the present invention ranges from 8 to 18 nm,
preferably from 10 to 17 nm, and still more preferably from 11 to
16.5 nm. The mean major axis length of the ferromagnetic powder
preferably ranges from 10 to 250 nm, more preferably from 15 to 150
nm, and still more preferably from 20 to 120 nm. The acicular ratio
of the ferromagnetic metal powder preferably ranges from 3 to 15,
more preferably from 3 to 10. The saturation magnetization
(.sigma.s) of the ferromagnetic metal powder preferably ranges from
90 to 170 A.multidot.m.sup.2 /kg, more preferably from 90 to 160
A.multidot.m.sup.2 /kg, and still more preferably from 100 to 160
A.multidot.m.sup.2 /kg. The coercive force of the ferromagnetic
metal powder preferably ranges from 135 to 279 kA/m, preferably
from 143 to 239 kA/m.
The moisture content of the ferromagnetic metal powder preferably
ranges from 0.1 to 2 percent. The moisture content of the
ferromagnetic powder is desirably optimized based on the type of
binder employed. The pH of the ferromagnetic powder is desirably
optimized based on the combination with the binder. The pH range
ranges from 6 to 12, preferably from 7 to 11. The SA (stearic acid)
adsorption capacity (a measure of the basicity of the surface) of
the ferromagnetic metal powder ranges from 1 to 15 .mu.mol/m.sup.2,
preferably from 2 to 10 .mu.mol/m.sup.2, and still more preferably
from 3 to 8 .mu.mol/m2. When a ferromagnetic metal powder with a
high capacity of stearic acid adsorption is employed, it is
desirable to modify the surface with organic matter strongly
adsorbing to the surface to produce a magnetic recording medium.
There are cases where soluble Na, Ca, Fe, Ni, Sr, NH.sub.4,
SO.sub.4, Cl, NO.sub.2, NO.sub.3, and other inorganic ions are
incorporated into the ferromagnetic metal powder. It is basically
desirable that these not be present, but they do not affect
characteristics so long as the total amount of each ion is about
300 ppm or less. Further, the ferromagnetic powder employed in the
present invention desirably has few pores, with the quantity
thereof preferably being equal to or less than 15 volume percent,
more preferably equal to or less than 5 volume percent. The shape
may be acicular, rice-particle shaped, or spindle-shaped so long as
the above-stated mean particle size and magnetic characteristics
are satisfied. A low SFD of the ferromagnetic powder itself is
desirable, and the Hc distribution of the ferromagnetic powder is
desirably narrowed. When the tape SFD is low, magnetization
reversal is sharp and peak shifts are small, which is suited to
high density digital magnetic recording. Methods of narrowing the
Hc distribution include improving the particle size distribution of
the goethite in the ferromagnetic metal powder, employing
monodispersed .alpha.-Fe.sub.2 O.sub.3, and preventing sintering
between particles.
Ferromagnetic Hexagonal Ferrite Powder
Barium ferrite, strontium ferrite, lead ferrite, calcium ferrite,
and various substitution products thereof, and Co substitution
products or the like, can be employed as the hexagonal ferrite
contained in the magnetic layer of the present invention. Specific
examples are magnetoplumbite-type barium ferrite and strontium
ferrite, magnetoplumbite-type ferrite, the particle surface of
which is covered with spinels, and composite magnetoplumbite-type
barium ferrite and strontium ferrite partly containing a spinel
phase. The following may be incorporated in addition to other
prescribed atoms: Al, Si, S, Nb, Sn, Ti, V, Cr, Cu, Y, Mo, Rh, Pd,
Ag, Sn, Sb, Te, W, Re, Au, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni,
B, Ge, Nb, and the like. Compounds to which elements such as
Co--Zn, Co--Ti, Co--Ti--Zr, Co--Ti--Zn, Ni--Ti--Zn, Nb--Zn--Co,
Sn--Zn--Co, Sn--Co--Ti, and Nb--Zn have been added may generally
also be employed. Specific impurities are also sometimes
incorporated based on the starting materials and manufacturing
method. Of these, magnetoplumbite-type hexagonal ferrite is
preferred because of its short wavelength output. The mean particle
size, measured either as a disk diameter or a hexagonal plate
diameter normally ranges from 10 to 50 nm, preferably from 10 to 45
nm, and more preferably from 10 to 40 nm.
Particularly when conducting reproduction with a magnetic
resistance (MR) head to improve track density, a plate diameter
equal to or less than 40 nm is desirable to reduce noise. However,
a plate diameter equal to or less than 10 nm is undesirable because
stable magnetization is difficult to achieve due to thermal
fluctuation. At greater than 50 nm, noise increases. Neither of
these cases is suited to high-density magnetic recording. A plate
ratio (plate diameter/plate thickness) ranging from 1 to 15 is
desirable, and from 1 to 7 is preferred. A low plate ratio is
undesirable because packing in the magnetic layer increases, but
making it difficult to achieve adequate orientation. Noise
increases due to stacking between particles at a plate ratio of
greater than 15. The S.sub.BET of the mean particle size normally
ranges from 30 to 200 m.sup.2 /g. The specific surface area is
generally coded as an arithmetic value calculated from the particle
plate diameter and the plate thickness. A narrower distribution of
the particle plate diameter and plate thickness is usually
preferred. To assign a number, comparison is possible by randomly
measuring 500 particles in a TEM (transmission electron microscope)
photograph of particles. Although the distribution is often not a
normal distribution, when calculated and denoted as the standard
deviation with respect to the mean particle size, it is given by
.sigma./mean particle size=0.1 to 2.0. To achieve a sharp particle
size distribution, the particle producing reaction system is
rendered as uniform as possible and the particles produced may be
subjected to a distribution-enhancing treatment. For example, one
known method is the graded dissolution of ultrafine particles in an
acid solution. In the vitrified crystal method, multiple rounds of
heat treatment are performed, and nucleus production and growth are
separated to achieve more uniform crystals. The coercivity Hc
measured in the magnetic powder can be made about 40 to 400 kA/m.
Although a high Hc is advantageous to high-density recording, this
is limited by the capacity of the recording head. The Hc can be
controlled through the particle size (plate diameter, plate
thickness), type and quantity of elements contained, substitution
site of elements, and the conditions under which the particle
generating reaction is conducted. Saturation magnetization .sigma.
s ranges from 30 to 70 A.multidot.m.sup.2 /kg. The .sigma. s tends
to decrease the smaller the particles become. Methods for
manufacturing include reducing the crystallization temperature
during manufacturing, reducing the temperature and/or duration of
the heat treatment, increasing the quantity of compounds added, and
increasing the level of surface treatment. It is also possible to
use W-type hexagonal ferrite. The magnetic material particle
surface is treated with a dispersion medium or substance suited to
the polymer in the course of dispersing the magnetic material. An
inorganic compound or organic compound is employed as the surface
treatment agent. Representative examples such compounds include
oxides or hydroxides of Si, Al, P, and Zr and the like as well as
various silane coupling agents and titanium coupling agents. The
quantity ranges from 0.1 to 10 percent with respect to the magnetic
material. The pH of the magnetic material is also important to
dispersion. A pH ranging from about 4 to 12 is usually optimal for
the dispersion medium and polymer, but a pH ranging from about 6 to
11 is selected for the chemical stability and storage properties of
the medium. Moisture contained in the magnetic material also
affects dispersion. Although there is an optimal value for the
dispersion medium and polymer, 0.1 to 2.0 mass percent is normally
selected. Methods of manufacturing hexagonal ferrite include: (1) a
vitrified crystallization method consisting of mixing into a
desired ferrite composition barium carbonate, iron oxide, and a
metal oxide substituting for iron with a glass forming substance
such as boron oxide; melting the mixture; rapidly cooling the
mixture to obtain an amorphous material; reheating the amorphous
material; and refining and comminuting the product to obtain a
barium ferrite crystal powder; (2) a hydrothermal reaction method
consisting of neutralizing a barium ferrite composition metal salt
solution with an alkali; removing the by-product; heating the
liquid phase to 100.degree. C. or greater; and washing, drying, and
comminuting the product to obtain barium ferrite crystal powder;
and (3) a coprecipitation method consisting of neutralizing a
barium ferrite composition metal salt solution with an alkali;
removing the by-product; drying the product and processing it at
equal to or less than 1,100.degree. C.; and comminuting the product
to obtain barium ferrite crystal powder. However, any manufacturing
method can be selected in the present invention.
The Lower Layer
The case where an essentially nonmagnetic lower layer (also
referred to as the "nonmagnetic layer") is present between the
support and the magnetic layer will be described next in detail.
The nonmagnetic layer normally comprises an inorganic powder.
Inorganic powders that may be selected for use are nonmagnetic
powders, examples of which are: metal oxides, metal carbonates,
metal nitrides, and metal carbides. Examples of inorganic compounds
are .alpha.-alumina having an .alpha.-conversion rate equal to or
higher than 90 percent, .beta.-alumina, .gamma.-alumina,
.theta.-alumina, silicon carbide, chromium oxide, cerium oxide,
.alpha.-iron oxide, goethite, silicon nitride, titanium dioxide,
silicon dioxide, tin oxide, magnesium oxide, zirconium oxide, zinc
oxide, and barium sulfate; these may be employed singly or in
combination. Particularly desirable due to their narrow particle
distribution and numerous means of imparting functions are titanium
dioxide, zinc oxide, .alpha.-iron oxide, goethite, and barium
sulfate. Even more preferred are titanium dioxide, .alpha.-iron
oxide, and goethite. The .alpha.-iron oxide is preferably in the
form of a magnetic iron oxide of uniform particle size or a
metal-use starting material that is dehydrated by heating, annealed
to reduce the number of pores, and surface treated when needed.
Titanium dioxide normally has photocatalytic properties, generating
radicals in the presence of light that run the risk of reacting
with the binder and lubricants. Thus, a 1 to 10 percent solid
solution of Al, Fe, or the like is desirably formed in the titanium
dioxide employed in the present invention to reduce the
photocatalytic characteristics. Further, the surface is desirably
treated with an Al and/or Si compound to reduce the catalytic
effect. The mean particle size of these nonmagnetic powders
preferably ranges from 5 to 1,000 nm, but nonmagnetic powders of
differing particle size may be combined as needed, or the particle
diameter distribution of a single nonmagnetic powder may be
broadened to achieve the same effect. What is preferred most is a
mean particle size in the nonmagnetic powder ranging from 10 to 500
nm. Particularly when the nonmagnetic powder is a granular metal
oxide, a mean particle diameter equal to or less than 80 nm is
preferred, and when an acicular metal oxide, a mean major axis
length equal to or less than 300 nm is preferred, with a particle
size equal to or less than 200 nm being even more preferable. The
tap density usually ranges from 0.3 to 1.5 g/mL, preferably from
0.4 to 1.3 g/mL. The moisture content of the nonmagnetic powder
usually ranges from 0.2 to 5 mass percent, preferably from 0.3 to 3
weight percent, and still more preferably from 0.3 to 1.5 weight
percent. The pH of the nonmagnetic power ranges from 2 to 12, with
a pH range of from 5.5 to 11 being particularly desirable. The
S.sub.BET of the nonmagnetic powder normally ranges from 1 to 150
m.sup.2 /g, preferably from 10 to 100 m.sup.2 /g, and still more
preferably from 20 to 100 m.sup.2 /g. The crystalline size of the
nonmagnetic powder preferably ranges from 4 to 100 nm, more
preferably from 4 to 80 nm. The dibutyl phthalate (DBP) oil
absorption capacity usually ranges from 5 to 100 mL/100 g,
preferably from 10 to 80 mL/100 g, and still more preferably from
20 to 60 mL/100 g. The specific gravity normally ranges from 1.5 to
7, preferably from 3 to 6. The shape may be any of acicular,
spherical, polyhedral, or plate-shaped. The SA (stearic acid)
absorption capacity of the nonmagnetic powder usually ranges from 1
to 20 .mu.mol/m.sup.2, preferably from 2 to 15 .mu.mol/m.sup.2, and
more preferably from 3 to 8 .mu.mol/m.sup.2. When employing a
nonmagnetic powder with a high capacity of stearic acid adsorption,
it is desirable to modify the surface with an organic substance
adsorbing strongly onto the surface to produce the magnetic
recording medium. The surfaces of these nonmagnetic powders are
preferably treated with compounds comprising elements such as Al,
Mg, Si, Ti, Zr, Sn, Sb, Zn, and Y. Compounds that are particularly
desirable as coating layers for their dispersion properties are
Al.sub.2 O.sub.3, SiO.sub.2, TiO.sub.2, ZrO.sub.2, MgO, and hydrous
oxides thereof. Even more preferable are Al.sub.2 O.sub.3,
SiO.sub.2, ZrO.sub.2, and hydrous oxides thereof. These may be
employed singly or in combination. Depending on the objective, a
surface-treated coating layer with a coprecipitated material may
also be employed, the coating structure which comprises a first
alumina coating and a second silica coating thereover or the
reverse structure thereof may also be adopted. Depending on the
objective, the surface-treated coating layer may be a porous layer,
but homogeneity and density are generally desirable.
Specific examples of nonmagnetic powders suitable for use in the
nonmagnetic layer of the magnetic recording medium of the present
invention are: Nanotite from Showa Denko K. K.; HIT-100 and HIT-80
from Sumitomo Chemical Co., Ltd.; .alpha.-iron oxide DPN 250BX,
DPN-245, DPN-270BX, DPN-550BX, DPN-550RX, DBN-450BX, DBN-650RX, and
DAN-850RX from Toda Kogyo Corp.; titanium oxide TTO-51B, TTO-55A,
TTO-55B, TTO-55C, TTO-55S, TTO-55D, and SN-100 from Ishihara Sangyo
Co., Ltd.; titanium oxide STT-4D, STT-30D, STT-30, STT-65C, and
.alpha.-iron oxide .alpha.-40 from Titan Kogyo K. K.; titanium
oxide MT-100S, MT-100T, MT-150W, MT-500B, MT-600B, MT-100F, and
MT-500HD from Tayca Corporation; FINEX-25, BF-1, BF-10, BF-20, and
ST-M from Sakai Chemical Industry Co., Ltd.; iron oxide DEFIC-Y and
DEFIC-R from Dowa Mining Co., Ltd.; AS2BM and TiO2P25 from Nippon
Aerogil; 100A and 500A from Ube Industries, Ltd.; and sintered
products of the same.
Mixing carbon black into the lower layer achieves the known effects
of lowering surface resistivity Rs and reducing light
transmittance, as well as yielding the desired micro Vickers
hardness. Further, the incorporation of carbon black into the
nonmagnetic layer can also serve to store lubricants. In that case,
the compound of general formula (I) above may be incorporated into
the nonmagnetic layer. Even when carbon black is not incorporated,
the nonmagnetic layer may comprise a compound having general
formula (I) above. Examples of types of carbon black that are
suitable for use are furnace black for rubber, thermal for rubber,
black for coloring, electrically conductive carbon black, and
acetylene black. Based on the effect desired, the following
characteristics can be optimized in the carbon black in the
nonmagnetic layer, and effects can be achieved by using different
carbon blacks in combination.
The S.sub.BET of the carbon black in the nonmagnetic layer normally
ranges from 50 to 500 m.sup.2 /g, preferably from 70 to 400 m.sup.2
/g. The DBP oil absorption capacity normally ranges from 20 to 400
mL/100 g, preferably from 30 to 400 mL/100 g. The mean particle
diameter of the carbon black normally ranges from 5 to 80 nm,
preferably from 10 to 50 nm, and more preferably from 10 to 40 nm.
Preferably, the pH of the carbon black ranges from 2 to 10, the
moisture content ranges from 0.1 to 10 percent, and the tap density
ranges from 0.1 to 1 g/mL. Specific examples of types of carbon
black suitable for use in the present invention are: BLACK PEARLS
2000, 1400, 1300, 1100, 1000, 900, 800, 880, 700, MONARCH 800, 880,
900, 1000, 1100, 1300, 1400, and VULCAN XC-72 from Cabot
Corporation; #9180, #3050B, #3150B, #3750B, #3950B, #2600, #2400,
#2350, #2300, #2200, #1000, #950, #650B, #970B, #850B, MA-600,
MA-230 #4000, and #4010 from Mitsubishi Chemical Corporation;
CONDUCTEX SC, RAVEN 8800, 8000, 7000, 5750, 5250, 3500, 2100, 2000,
1800, 1500, 1255, 1250, 2500 ULTRA, 5000, 5000 ULTRA II, 5000 ULTRA
III, and 1200 from Columbia Carbon Co., Ltd.; and Ketjen Black EC
from Lion Akzo Co., Ltd. The carbon black employed can be surface
treated with a dispersing agent or the like, grafted with a resin,
or a portion of the surface may be graphite-treated. Further, the
carbon black may be dispersed with a binder prior to being added to
the coating material. These types of carbon black are employed in a
range that does not exceed 50 mass percent with respect to the
inorganic powder above and does not exceed 40 percent with respect
to the total mass of the nonmagnetic layer. These types of carbon
black may be employed singly or in combination. The Carbon Black
Handbook compiled by the Carbon Black Association may be consulted
for types of carbon black suitable for use in the present
invention.
Based on the objective, an organic powder may be added to the
nonmagnetic layer. Examples are acrylic styrene resin powders,
benzoguanamine resin powders, melamine resin powders, and
phthalocyanine pigments. Polyolefin resin powders, polyester resin
powders, polyamide resin powders, polyimide resin powders, and
polyfluoroethylene resins may also be employed. The manufacturing
methods described in Japanese-Unexamined Patent Publication (KOKAI)
Showa Nos. 62-18564 and 60-255827 may be employed.
As regards the binder resin (type and quantity); quantity and type
of lubricants, dispersants, and additives; solvents; and dispersion
methods of the nonmagnetic layer, the techniques known with regard
to magnetic layers may be applied.
The Binder
Conventionally known thermoplastic resins, thermosetting resins,
reactive resins and mixtures thereof may be employed as binders to
form the magnetic layer, and if desired, the nonmagnetic layer and
backcoating layer of the present invention. The thermoplastic
resins have a glass transition temperature of from -100 to
150.degree. C., normally have a number average molecular weight of
from 1,000 to 200,000, preferably from 10,000 to 100,000, and have
a degree of polymerization of about from 50 to 1,000. Examples are
polymers and copolymers comprising structural units in the form of
vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic
acid, acrylic acid esters, vinylidene chloride, acrylonitrile,
methacrylic acid, methacrylic acid esters, styrene, butadiene,
ethylene, vinyl butyral, vinyl acetal, and vinyl ether;
polyurethane resins; and various rubber resins.
Further, examples of thermosetting resins and reactive resins are
phenol resins, epoxy resins, polyurethane cured resins, urea
resins, melanine resins, alkyd resins, acrylic reactive resins,
formaldehyde resins, silicone resins, epoxy polyamide resins,
mixtures of polyester resins and isocyanate prepolymers, mixtures
of polyester polyols and polyisocyanates, and mixtures of
polyurethane and polyisocyanates. These resins are described in
detail in the Handbook of Plastics published by Asakura Shoten. It
is also possible to employ known electron beam-cured resins in
individual layers. Examples thereof and methods of manufacturing
the same are described in detail in Japanese Unexamined Patent
Publication (KOKAI) Showa No. 62-256219. The above-listed resins
may be used singly or in combination. Preferred resins are
combinations of polyurethane resin and at least one member selected
from the group consisting of vinyl chloride resin, vinyl
chloride--vinyl acetate copolymers, vinyl chloride--vinyl
acetate--vinyl alcohol copolymers, and vinyl chloride--vinyl
acetate--maleic anhydride copolymers, as well as combinations of
the same with polyisocyanate.
Known structures of polyurethane resin can be employed, such as
polyester polyurethane, polyether polyurethane, polyether polyester
polyurethane, polycarbonate polyurethane, polyester polycarbonate
polyurethane, and polycaprolactone polyurethane.
Of these, polyurethane resin comprising diol components in the form
of a short-chain diol with a molecular weight of less than 500
having a cyclic structure and a long-chain polyetherdiol having a
molecular weight of from 500 to 5,000 are preferred.
The short-chain diol having a molecular weight of less than 500 and
a cyclic structure (referred to hereinafter simply as "short-chain
diol") is preferably selected from among aromatic and aliphatic
diols, and ethylene oxide or propylene oxide adducts thereof and
the like.
Examples of short-chain diols are bisphenol A, bisphenol A hydride,
bisphenol S, bisphenol S hydride, bisphenol P, bisphenol P hydride,
cyclohexanedimethanol, cyclohexanediol, and hydroquinone. Of these,
bisphenol A, bisphenol A hydride, and ethylene oxide adducts and
propylene oxide adducts of the same are preferred. Of even greater
preference is bisphenol A hydride. The content of the short-chain
diol in the polyurethane resin preferably ranges from 15 to 40 mass
percent.
Preferred examples of long-chain polyether diols having a molecular
weight ranging from 500 to 5,000 (referred to hereinafter simply as
"long-chain diols") are propylene oxide adducts of bisphenol A,
ethylene oxide adducts of bisphenol A, ethylene oxide adducts of
bisphenol A hydride, and propylene oxide adducts of bisphenol A
hydride.
Polyurethane comprising polyisocyanate and dimer-diol obtained by
converting into a dimeric acid in the form of an unsaturated
aliphatic carboxylic acid with 18 carbon atoms, adding hydrogen to
reduce the unsaturated bond and the carboxylic acid, and refining
the product by distillation is preferred.
To obtain better dispersability and durability in all of the
binders set forth above, it is desirable to introduce by
copolymerization or addition reaction one or more polar groups
selected from among --COOM, --SO.sub.3 M, --OSO.sub.3 M,
--P.dbd.O(OM).sub.2, --O--P.dbd.O(OM).sub.2, (where M denotes a
hydrogen atom or an alkali metal), OH, NR.sub.2, N.sup.+ R.sub.3
(where R denotes a hydrocarbon group), epoxy groups, SH, and CN.
The quantity of the polar group is from 10.sup.-1 to 10.sup.-8
mol/g, preferably from 10.sup.-2 to 10.sup.-6 mol/g.
Specific examples of the binders employed in the present invention
are VAGH, VYHH, VMCH, VAGF, VAGD, VROH, VYES, VYNC, VMCC, XYHL,
XYSG, PKHH, PKHJ, PKHC, and PKFE from Union Carbide Corporation;
MPR-TA, MPR-TA5, MPR-TAL, MPR-TSN, MPR-TMF, MPR-TS, MPR-TM, and
MPR-TAO from Nisshin Kagaku Kogyo K. K.; 1000W, DX80, DX81, DX82,
DX83, and 100FD from Denki Kagaku Kogyo K. K.; MR-104, MR-105,
MR110, MR100, MR555, and 400X-110A from Nippon Zeon Co., Ltd.;
Nippollan N2301, N2302, and N2304 from Nippon Polyurethane Co.,
Ltd.; Pandex T-5105, T-R3080, T-5201, Burnock D-400, D-210-80,
Crisvon 6109, and 7209 from Dainippon Ink and Chemicals
Incorporated.; Vylon UR8200, UR8300, UR-8700, RV530, and RV280 from
Toyobo Co., Ltd.; Daipheramine 4020, 5020, 5100, 5300, 9020, 9022,
and 7020 from Dainichiseika Color & Chemicals Mfg. Co., Ltd.;
MX5004 from Mitsubishi Chemical Corporation; Sanprene SP-150 from
Sanyo Chemical Industries, Ltd.; and Saran F310 and F210 from Asahi
Chemical Industry Co., Ltd.
The binder employed in the nonmagnetic layer and magnetic layer of
the magnetic recording medium obtained based on the present
invention is employed in a range of from 5 to 50 mass percent,
preferably from 10 to 30 mass percent with respect to the
nonmagnetic powder in the nonmagnetic layer and the magnetic powder
in the magnetic layer. Vinyl chloride resin, polyurethane resin,
and polyisocyanate are preferably combined within the ranges of: 5
to 30 mass percent for vinyl chloride resin, when employed; 2 to 20
mass percent for polyurethane resin, when employed; and 2 to 20
mass percent for polyisocyanate. However, when a small amount of
dechlorination causes head corrosion, for example, it is also
possible to employ polyurethane alone, or employ polyurethane and
isocyanate alone. In the present invention, when polyurethane is
employed, a glass transition temperature of from -50 to 150.degree.
C., preferably from 0 to 100.degree. C., an elongation at break of
from 100 to 2,000 percent, a stress at break of from 0.05 to 10
kg/mm.sup.2 (0.49 to 98 MPa), and a yield point of from 0.05 to 10
kg/mm.sup.2 (0.49 to 98 MPa) are desirable.
The magnetic recording medium according to the present invention
comprises at least two layers. Accordingly, the quantity of binder;
the quantity of vinyl chloride resin, polyurethane resin,
polyisocyanate, or some other resin in the binder; the molecular
weight of each of the resins forming the magnetic layer; the
quantity of polar groups; or the physical characteristics or the
like of the above-described resins can naturally be different in
each of the layers as required. These are optimized in each layer.
Known techniques may be applied for a multilayered magnetic layer.
For example, when the quantity of binder is different in each
layer, increasing the quantity of binder in the magnetic layer
effectively decreases scratching on the surface of the magnetic
layer. To achieve good head touch, the quantity of binder in the
nonmagnetic layer can be increased to impart flexibility.
Examples of polyisocyanates suitable for use in the magnetic layer,
nonmagnetic layer, and backcoating layer of the present invention
are tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate,
hexamethylene diisocyanate, xylylene diisocyanate,
napthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophorone
diisocyanate, triphenylmethane triisocyanate, and other
isocyanates; products of these isocyanates and polyalcohols;
polyisocyanates produced by condensation of isocyanates; and the
like. These isocyanates are commercially available under the
following trade names, for example: Coronate L, Coronate HL,
Coronate 2030, Coronate 2031, Millionate MR and Millionate MTL
manufactured by Nippon Polyurethane Industry Co. Ltd.; Takenate
D-102, Takenate D-110N, Takenate D-200 and Takenate D-202
manufactured by Takeda Chemical Industries Co., Ltd.; and Desmodule
L, Desmodule IL, Desmodule N and Desmodule HL manufactured by
Sumitomo Bayer Co., Ltd. They can be used singly or in combinations
of two or more in all layers by exploiting differences in curing
reactivity.
Carbon Black
Carbon black may be incorporated into the magnetic layer; in that
case, the compounds of general formula (I) above may be employed in
the magnetic layer. However, even when carbon black is not
employed, the compound of general formula (I) above may still be
incorporated into the magnetic layer. Examples of types of carbon
black that are suitable for use are: furnace black for rubber,
thermal for rubber, black for coloring, electrically conductive
carbon black, and acetylene black. A specific surface area of from
5 to 500 m.sup.2 /g, a DBP oil absorption capacity of from 10 to
400 mL/100 g, a mean particle diameter of from 5 to 300 nm, a pH of
from 2 to 10, a moisture content of from 0.1 to 10 mass percent,
and a tap density of from 0.1 to 1 g/mL are desirable. Specific
examples of types of carbon black employed in the present invention
are: BLACK PEARLS 2000, 1300, 1000, 900, 880, 800, 700 and VULCAN
XC-72 from Cabot Corporation; #80, #60, #55, #50 and #35
manufactured by Asahi Carbon Co., Ltd.; #2400B, #2300, #900, #1000,
#30, #40 and #10B from Mitsubishi Chemical Corporation; CONDUCTEX
SC, RAVEN 150, 50, 40, 15 and RAVEN MT-P from Columbia Carbon Co.,
Ltd.; and Ketjen Black EC from Lion Akzo Co., Ltd. The carbon black
employed may be surface-treated with a dispersant or grafted with
resin, or have a partially graphite-treated surface. The carbon
black may be dispersed in advance into the binder prior to addition
to the magnetic coating material. These carbon blacks may be used
singly or in combination. When employing carbon black, the quantity
preferably ranges from 0.1 to 30 mass percent with respect to the
magnetic material. In the magnetic layer, carbon black works to
prevent static, reduce the coefficient of friction, impart
light-blocking properties, enhance film strength, and the like; the
properties vary with the type of carbon black employed.
Accordingly, the type, quantity, and combination of carbon blacks
employed in the present invention may be determined separately for
the upper magnetic layer and the lower nonmagnetic layer based on
the objective and the various characteristics stated above, such as
particle size, oil absorption capacity, electrical conductivity,
and pH, and be optimized for each layer. For example, the Carbon
Black Handbook compiled by the Carbon Black Association may be
consulted for types of carbon black suitable for use in the
magnetic layer of the magnetic recording medium of the present
invention.
Abrasives
Known materials, chiefly with a Mohs' hardness equal to or higher
than 6, such as .alpha.-alumina having an .alpha.-conversion rate
equal to or higher than 90 percent, .beta.-alumina, microgranular
diamond, silicon carbide, chromium oxide, cerium oxide,
.alpha.-iron oxide, corundum, silicon nitride, titanium carbide,
titanium oxide, silicon dioxide, and boron nitride, may be used
singly or in combination as abrasives in the magnetic layer of the
magnetic recording medium of the present invention. Further, a
composite comprising two or more of these abrasives (an abrasive
obtained by surface-treating one abrasive with another) may also be
used. Although these abrasives may contain compounds and elements
other than the main component or element in some cases, there is no
change in effect so long as the main component constitutes equal to
or higher than 90 mass percent. The mean particle size of these
abrasives preferably ranges from 10 to 1,000 nm, a narrow particle
size distribution being particularly desirable for improving
electromagnetic characteristics. As needed to improve durability,
abrasives of differing particle size may be combined or the same
effect may be achieved by broadening the particle diameter
distribution even with a single abrasive. A tap density of from 0.3
to 1.5 g/mL, a moisture content of from 0.1 to 5 mass percent, a pH
of from 2 to 11, and a specific surface area of from 1 to 40
m.sup.2 /g are desirable. The abrasive employed in the present
invention may be any of acicular, spherical, or cubic in shape, but
shapes that are partially angular have good abrasion properties and
are thus preferred. Specific examples: AKP-10, AKP-15, AKP-20,
AKP-30, AKP-50, HIT-20, HIT-30, HIT-50, HIT-50G, HIT-60A, HIT-60G,
HIT-70, HIT-80, HIT-82, and HIT-100 from Sumitomo Chemical Co.,
Ltd.; ERC-DBM, HP-DBM, and HPS-DBM from Reynolds Co.; WA10000 from
Fujimi Abrasives Co.; UB20 from Kamimura Kogyo Co., Ltd.; G-5,
Chromex U2, and Chromex U1 from Nippon Chemical Industrial Co.,
Ltd.; TF100 and TF-140 from Toda Kogyo Corp.; Beta Random Ultrafine
from Ibidene Co.; and B-3 from Showa Mining Co., Ltd. As needed,
these abrasives may be added to the nonmagnetic layer. Addition to
the nonmagnetic layer permits control of surface shape and control
of the manner in which the abrasive protrudes. The particle
diameter and quantity of abrasive added to the magnetic layer and
nonmagnetic layer are optimally established, as a matter of course.
An effective manufacturing method is to separately disperse the
abrasives, and then add them to the magnetic layer or nonmagnetic
layer coating material.
Additives
Substances having lubricating effects, antistatic effects,
dispersive effects, plasticizing effects, or the like may be
employed as additives in the magnetic layer and nonmagnetic layer
of the magnetic recording medium according to the present
invention. The following are suitable for use: molybdenum
disulfide; tungsten disulfide; graphite; boron nitride; graphite
fluoride; silicone oils; silicones having a polar group; fatty
acid-modified silicones; fluorine-containing silicones;
fluorine-containing alcohols; fluorine-containing esters;
polyolefins; polyglycols; alkylphosphoric esters and their alkali
metal salts; alkylsulfuric esters and their alkali metal salts;
polyphenyl ethers; phenylphosphonic acid;
.alpha.-naphthylphosphoric acid; phenylphosphoric acid;
diphenylphosphoric acid; p-ethylbenzenephosphonic acid;
phenylphosphinic acid; aminoquinones; various silane coupling
agents and titanium coupling agents; fluorine-containing alkyl
sulfuric acid esters and their alkali metal salts; monobasic fatty
acids (which may contain an unsaturated bond or be branched) having
10 to 24 carbon atoms and metal salts (such as Li, Na, K, and Cu)
thereof; monohydric, dihydric, trihydric, tetrahydric, pentahydric
or hexahydric alcohols with 12 to 22 carbon atoms (which may
contain an unsaturated bond or be branched); alkoxy alcohols with
12 to 22 carbon atoms (which may contain an unsaturated bond or be
branched); monofatty esters, difatty esters, or trifatty esters
comprising a monobasic fatty acid having 10 to 24 carbon atoms
(which may contain an unsaturated bond or be branched) and any one
from among a monohydric, dihydric, trihydric, tetrahydric,
pentahydric or hexahydric alcohol having 2 to 12 carbon atoms
(which may contain an unsaturated bond or be branched); fatty acid
esters of monoalkyl ethers of alkylene oxide polymers; fatty acid
amides with 8 to 22 carbon atoms; and aliphatic amines with 8 to 22
carbon atoms.
Specific examples of these fatty acids are: capric acid, caprylic
acid, lauric acid, myristic acid, palmitic acid, stearic acid,
behenic acid, oleic acid, elaidic acid, linolic acid, linolenic
acid, and isostearic acid. Examples of fatty acid esters are butyl
stearate, octyl stearate, amyl stearate, isooctyl stearate, butyl
myristate, octyl myristate, butoxyethyl stearate, butoxydiethyl
stearate, 2-ethylhexyl stearate, 2-octyldodecyl palmitate,
2-hexyldodecyl palmitate, isohexadecyl stearate, oleyl oleate,
dodecyl stearate, tridecyl stearate, oleyl erucate, neopentylglycol
didecanoate, and ethylene glycol dioleyl. Examples of alcohols are
oleyl alcohol, stearyl alcohol, and lauryl alcohol. It is also
possible to employ nonionic surfactants such as alkylene
oxide-based surfactants, glycerin-based surfactants, glycidol-based
surfactants and alkylphenolethylene oxide adducts; cationic
surfactants such as cyclic amines, ester amides, quaternary
ammonium salts, hydantoin derivatives, heterocycles, phosphoniums,
and sulfoniums; anionic surfactants comprising acid groups, such as
carboxylic acid, sulfonic acid, phosphoric acid, sulfuric ester
groups, and phosphoric ester groups; and ampholytic surfactants
such as amino acids, amino sulfonic acids, sulfuric or phosphoric
esters of amino alcohols, and alkyl betaines. Details of these
surfactants are described in A Guide to Surfactants (published by
Sangyo Tosho K. K.). These lubricants, antistatic agents and the
like need not be 100 percent pure and may contain impurities, such
as isomers, unreacted material, by-products, decomposition
products, and oxides in addition to the main components. These
impurities preferably comprise equal to or less than 30 mass
percent, and more preferably equal to or less than 10 mass
percent.
The lubricants and surfactants employed in the present invention
each have different physical effects. The type, quantity, and
combination ratio of lubricants producing synergistic effects are
optimally set for a given objective. It is conceivable to control
bleeding onto the surface through the use of fatty acids having
different melting points in the nonmagnetic layer and the magnetic
layer; to control bleeding onto the surface through the use of
esters having different boiling points, melting points, and
polarity; to improve the stability of coatings by adjusting the
quantity of surfactant; and to increase the lubricating effect by
increasing the amount of lubricant in the nonmagnetic layer. The
present invention is not limited to these examples. Generally, a
total quantity of lubricant ranging from 0.1 to 50 mass percent,
preferably from 2 to 25 mass percent with respect to the magnetic
material or nonmagnetic powder is selected.
All or some of the additives used in the present invention may be
added at any stage in the process of manufacturing the magnetic
layer and nonmagnetic layer coating liquids. For example, they may
be mixed with the magnetic material before a kneading step; added
during a step of kneading the magnetic material, the binder, and
the solvent; added during a dispersing step; added after
dispersing; or added immediately before coating. Part or all of the
additives may be applied by simultaneous or sequential coating
after the magnetic layer has been applied to achieve a specific
purpose. Depending on the objective, the lubricant may be coated on
the surface of the magnetic layer after calendering or making
slits.
Known organic solvents may be employed in the present invention.
For example, the solvents described in Japanese Unexamined Patent
Publication (KOKAI) Showa No. 6-68453 may be employed.
The Layer Structure
In the thickness structure of the magnetic recording medium
according to the present invention, in the case of a magnetic tape,
the support is 2.5 to 20 .mu.m in thickness, and to increase the
volume density, 2.5 to 10 .mu.m, preferably 2.5 to 8 .mu.m. In the
case of a flexible magnetic disk, the support is 20 to 100 .mu.m in
thickness, preferably 20 to 75 .mu.m.
An undercoating layer may be provided to improve adhesion between
the support and the nonmagnetic layer or magnetic layer. The
thickness of the undercoating layer ranges from 0.01 to 0.5 .mu.m,
preferably from 0.02 to 0.5 .mu.m. Known undercoating layers may be
employed.
The thickness of the magnetic layer of the medium according to the
present invention is optimized based on the saturation
magnetization level of the head, the head gap length, and the
recording signal band. A thickness of from 0.04 to 0.3 .mu.m,
preferably from 0.04 to 0.25 .mu.m is generally suitable. It is
also possible to separate the magnetic layer into two or more
layers having different magnetic characteristics. Known multiple
magnetic layer structures may be employed. When two or more
magnetic layers are provided, the "thickness of the magnetic layer"
refers to the thickness of the uppermost layer.
The thickness of the nonmagnetic layer on which the magnetic layer
is provided ranges from 0.2 to 5.0 .mu.m, preferably from 0.3 to
3.0 .mu.m, and still more preferably from 0.5 to 2.5 .mu.m. The
nonmagnetic layer performs its function so long as it is
essentially nonmagnetic. For example, the presence or intentional
incorporation of a small quantity of impurity is permissible. The
term "essentially nonmagnetic" means that the nonmagnetic layer
exhibits a residual magnetic flux density equal to or less than 50
mT or a coercive force equal to or less than 40 percent than that
of the magnetic layer, preferably exhibiting no residual magnetic
flux density or coercive force at all.
The Backcoating Layer
Generally, in magnetic tapes for computer data recording, greater
repeat running properties are demanded than is the case for video
tapes and audio tapes. To maintain such high running durability, a
backcoating layer is desirably provided on the opposite side from
the magnetic layer in the present invention. The backcoating layer
functions to prevent static and compensate for curling.
The backcoating layer preferably comprises carbon black and the
above-described compound of general formula (I) dispersed in a
binder described below. However, the addition of other optional
components in the form of dispersing agents and lubricants is
desirable. Examples of dispersing agents are: fatty acids having 12
to 18 carbon atoms (RCOOH, R denoting an alkenyl group or an alkyl
group having 11 to 17 carbon atoms) such as caprylic acid, capric
acid, lauric acid, myristic acid, palmitic acid, stearic acid,
behenic acid, oleic acid, elaidic acid, linolic acid, linolenic
acid, and stearolic acid; metallic soaps of the above-listed fatty
acids with alkali metals or alkaline earth metals;
fluorine-containing compounds of the above-listed fatty acid
esters; amides of the above-listed fatty acids; polyalkyleneoxide
alkylphosphoric acid ester; lecithin; trialkylpolyolefinoxy
quaternary ammonium salts (the alkyls having 1 to 5 carbon atoms
and the olefin being ethylene, propylene, or the like); sulfuric
acid esters; and copper phthalocyanine derivatives other than the
compound of above-recorded general formula (I). These may be
employed singly or in combination. Of these, copper oleate, copper
phthalocyanine derivatives other than the above-recorded compound
of general formula (I), and barium sulfate are preferred. The
dispersant is added within a range of from 0.5 to 20 mass parts per
100 mass parts of the binder resin.
The lubricant employed may be selected from among the lubricants
that are commonly employed in conventional magnetic tapes. However,
in the present invention, fatty acids having equal to or higher
than 18 carbon atoms, or fatty acid esters, are preferred from the
viewpoint of improving running properties. The lubricant is
normally added in a proportion of from 1 to 5 mass parts per 100
mass parts of binder resin.
Examples of binders used to form the backcoating layer of the
present invention are thermoplastic resins, thermosetting resins,
reactive resins, and mixtures thereof. Examples of thermoplastic
resins are: polyvinyl chloride resin, polyurethane resin, phenoxy
resin, vinyl chloride--vinyl acetate copolymers, vinyl
chloride--vinylidene chloride copolymers, vinyl
chloride--acrylonitrile copolymers, acrylic acid
ester--acrylonitrile copolymers, acrylic acid ester--vinylidene
chloride copolymers, acrylic acid ester--styrene copolymers,
methacrylic acid ester--acrylonitrile copolymers, methacrylic acid
ester--vinylidene chloride copolymers, methacrylic acid
ester--styrene copolymers, polyvinyl fluoride, vinylidene
chloride--acrylonitrile copolymers, butadiene--acrylonitrile
copolymers, polyamide resins, polyvinyl butyral, cellulose resins
(cellulose acetate butyrate, cellulose diacetate, cellulose
propionate, nitrocellulose, and the like), styrene--butadiene
copolymers, polyester resins, chlorovinyl ether--acrylic acid ester
copolymers, amino resins, and various rubber resins. Examples of
thermosetting resins and reactive resins are: phenol resins, epoxy
resins, polyurethane cured resins, urea resins, melamine resins,
alkyd resins, acrylic reactive resins, formaldehyde resins,
silicone resins, epoxy-polyamide resins, and polyisocyanates.
The Support
Known films of the following may be employed as the support in the
present invention: polyesters such as polyethylene terephthalate,
polyethylene naphthalate, polyolefins, cellulose triacetate,
polycarbonate, polyamides comprising aromatic polyamides such as
aramides, polyimides, polyamidoimides, polysulfones, and
polybenzooxazole. A support having a glass transition temperature
equal to or higher than 100.degree. C., particularly from 120 to
400.degree. C., is preferred. The use of high-strength support such
as polyethylene naphthalate (PEN), polyamides is particularly
desirable. As needed, stacked supports such as are disclosed in
Japanese Unexamined Patent Publication (KOKAI) Heisei No. 3-224127
may be employed to vary the surface roughness of the magnetic
surface and support surface. These supports may be subjected
beforehand to corona discharge treatment, plasma treatment,
adhesion enhancing treatment, heat treatment, dust removal, and the
like.
To achieve the object of the present invention, the use of a
support with a center surface average surface roughness (SRa) as
measured by optical interference roughness meter equal to or less
than 8.0 nm, preferably equal to or less than 4.0 nm, and still
more preferably equal to or less than 2.0 nm, is desirable. It is
desirable that the support simply has not only a low center surface
average surface roughness, but also no coarse protrusions of 0.5
.mu.m or above. The shape of the roughness of the surface may be
controlled as needed through the size and quantity of filler that
is added to the support. Examples of these fillers are oxides and
carbonates of Ca, Si, Ti and the like; and acrylic-based organic
powders. Preferably, the maximum height of the support, SR.sub.max,
is equal to or less than 1 .mu.m; the ten-point average roughness
SR.sub.z is equal to or less than 0.5 .mu.m; the center surface
peak height SR.sub.p is equal to or less than 0.5 .mu.m, the center
surface valley depth SR.sub.v is equal to or less than 0.5 .mu.m,
the center surface area percentage SS.sub.r is equal to or higher
than 10 percent and equal to or less than 90 percent, and the
average wavelength S.lambda. a is equal to or higher than 5 .mu.m
and equal to or less than 300 .mu.m. The surface protrusion
distribution of the support may be controlled at will with filler
to achieve desired electromagnetic characteristics and durability.
The number of each surface protrusion ranging from 0.01 .mu.m to 1
.mu.m in size per 0.1 mm.sup.2 may be controlled to within a range
of from 0 to 2,000.
The F-5 value of the support employed in the present invention
preferably ranges from 5 to 50 kg/mm.sup.2 (49 to 490 MPa) and the
thermal shrinkage rate of the support after 30 minutes at
100.degree. C. is preferably equal to or less than 3 percent, more
preferably equal to or less than 1.5 percent. The thermal shrinkage
rate after 30 min at 80.degree. C. is equal to or less than 1
percent, preferably equal to or less than 0.5 percent. A breaking
strength of 5 to 100 kg/mm.sup.2 (49 to 980 MPa) and a modulus of
elasticity of from 100 to 2,000 kg/mm.sup.2 (980 to 19600 MPa) are
preferred. The coefficient of thermal expansion is from 10.sup.-4
to 10.sup.-8 /.degree.C., preferably from 10.sup.-5 to 10.sup.-6
/.degree.C. The coefficient of moisture expansion is equal to or
less than 10.sup.-4 /RH percent, preferably equal to or less than
10.sup.-5 /RH percent. These thermal characteristics, dimensional
characteristics, and mechanical strength characteristics are
preferably nearly equal, differing by 10 percent or less, in any
in-plane direction of the support.
Manufacturing Method
The process for manufacturing the magnetic coating material and the
nonmagnetic coating material of the magnetic recording medium
according to the present invention comprises at least a kneading
step, a dispersing step, and a mixing step to be carried out, if
necessary, before and/or after the kneading and dispersing steps.
Each of the individual steps may be divided into two or more
stages. All of the starting materials employed in the present
invention, including the magnetic material, nonmagnetic powder,
binders, carbon black, abrasives, antistatic agents, lubricants,
solvents, and the like, may be added at the beginning of, or
during, any of the steps. Moreover, the individual starting
materials may be divided up and added during two or more steps. For
example, polyurethane may be divided up and added in the kneading
step, the dispersion step, and the mixing step for viscosity
adjustment after dispersion. To achieve the object of the present
invention, conventionally known manufacturing techniques may be
utilized for some of the steps. A kneader having a strong kneading
force, such as an open kneader, continuous kneader, pressure
kneader, or extruder is preferably employed in the kneading step.
When a kneader is employed, the magnetic material or nonmagnetic
powder and all or part of the binder (preferably equal to or higher
than 30 mass percent of the entire quantity of binder) are kneaded
in a range of from 15 to 500 parts per 100 parts of magnetic
material. Details of the kneading process are described in Japanese
Unexamined Patent Publication (KOKAI) Heisei Nos. 1-106338 and
1-79274. The nonmagnetic layer coating material may also be
adjusted based on the quantity of magnetic coating material.
Further, glass beads may be employed to disperse the magnetic
coating material and nonmagnetic coating material, with a
dispersing medium with a high specific gravity such as zirconia
beads, titania beads, and steel beads being suitable for use. The
particle diameter and fill ratio of these dispersing media are
optimized for use. A known dispersing device may be employed.
Magnetic material, abrasives, and carbon black having different
dispersion rates may first be separately dispersed, admixed, and
then microdispersed if needed, to obtain coating solutions.
Methods such as the following are desirably employed when coating a
multilayer structure magnetic recording medium in the present
invention. In the first method, the nonmagnetic layer is first
applied with a coating device commonly employed to apply magnetic
coating materials such as a gravure coating, roll coating, blade
coating, or extrusion coating device, and the upper layer is
applied while the nonmagnetic layer is still wet by means of a
support pressure extrusion coating device such as is disclosed in
Japanese Examined Patent Publication (KOKOKU) Heisei No. 1-46186
and Japanese Unexamined Patent Publication (KOKAI) Showa No.
60-238179 and Japanese Unexamined Patent Publication (KOKAI) Heisei
No. 2-265672. In the second method, the upper and lower layers are
applied nearly simultaneously by a single coating head having two
built-in slits for passing coating solution, such as is disclosed
in Japanese Unexamined Patent Publication (KOKAI) Showa No.
63-88080, Japanese Unexamined Patent Publication (KOKAI) Heisei No.
2-17971, and Japanese Unexamined Patent Publication (KOKAI) Heisei
No. 2-265672. In the third method, the upper and lower layers are
applied nearly simultaneously using an extrusion coating apparatus
with a backup roller as disclosed in Japanese Unexamined Patent
Publication (KOKAI) Heisei No. 2-174965. To avoid compromising the
electromagnetic characteristics or the like of the magnetic
recording medium by aggregation of magnetic particles, shear is
desirably imparted to the coating liquid in the coating head by a
method such as disclosed in Japanese Unexamined Patent Publication
(KOKAI) Showa No. 62-95174 or Japanese Unexamined Patent
Publication (KOKAI) Heisei No. 1-236968. In addition, the viscosity
of the coating liquid must satisfy the numerical range specified in
Japanese Unexamined Patent Publication (KOKAI) Heisei No. 3-8471.
Applying the nonmagnetic layer, drying it, and then applying the
magnetic layer thereover in a sequential multilayer coating to
achieve the structure of the magnetic recording medium of the
present invention is also possible, and does not compromise the
effect of the present invention. However, to reduce the number of
voids in the coating and improve the quality as regards dropout and
the like, the above-describe simultaneous multilayer coating is
preferred.
Processing with calender processing rollers in the form of
heat-resistant plastic rollers of epoxy, polyimide, polyamide,
polyimidoamide or the like, or metal rollers, is desirable. The
processing temperature is preferably equal to or higher than
50.degree. C., more preferably equal to or higher than 100.degree.
C. Linear pressure is desirably equal to or higher than 200 kg/cm
(196 kN/m), more preferably equal to or higher than 300 kg/cm (294
kN/m).
It is preferable that the coefficient of friction of the magnetic
recording medium of the present invention relative to the head is
equal to or less than 0.5 and preferably equal to or less than 0.3
at temperatures ranging from -10.degree. to 40.degree. C. and
humidity ranging from 0 percent to 95 percent, the specific surface
resistivity desirably ranges from 10.sup.4 to 10.sup.12 .OMEGA./sq
on the magnetic surface, and the charge potential preferably ranges
from -500 V to +500 V. The modulus of elasticity at 0.5 percent
extension of the magnetic layer desirably ranges from 100 to 2,000
kg/mm.sup.2 (980 to 1,960 MPa) in each in-plane direction. The
breaking strength desirably ranges from 10 to 70 kg/mm.sup.2 (98 to
686 MPa). The modulus of elasticity of the magnetic recording
medium desirably ranges from 100 to 1,500 kg/mm.sup.2 (980 to
14,700 MPa) in each in-plane direction. The residual elongation is
desirably equal to or less than 0.5 percent, and the thermal
shrinkage rate at all temperatures below 100.degree. C. is
preferably equal to or less than 1 percent, more preferably equal
to or less than 0.5 percent, and most preferably equal to or less
than 0.1 percent. The glass transition temperature of the magnetic
layer (i.e., the temperature at which the loss elastic modulus of
dynamic viscoelasticity peaks as measured at 110 Hz) of the
magnetic layer is preferably equal to or higher than 50.degree. C.
and equal to or less than 120.degree. C., and that of the lower
nonmagnetic layer preferably ranges from 0 to 100.degree. C. The
loss elastic modulus preferably falls within a range of from
1.times.10.sup.7 to 8.times.10.sup.8 N/m.sup.2 and the loss tangent
is preferably equal to or less than 0.2. Adhesion failure tends to
occur when the loss tangent becomes excessively large. These
thermal characteristics and mechanical characteristics are
desirably nearly identical, varying by 10 percent or less, in each
in-plane direction of the medium. The residual solvent in the
magnetic layer is preferably equal to or less than 100 mg/m.sup.2
and more preferably equal to or less than 10 mg/m.sup.2. The void
ratio in the coated layers, including both the nonmagnetic layer
and the magnetic layer, is preferably equal to or less than 30
volume percent, more preferably equal to or less than 20 volume
percent. Although a low void ratio is preferable for attaining high
output, there are some cases in which it is better to ensure a
certain level based on the object.
The surface roughness of the magnetic layer surface, in the form of
the center surface average surface roughness Ra as measured by
optical interference roughness meter, preferably ranges from 1.0 to
3.0 nm, more preferably equal to or less than 2.8 nm, and still
more preferably equal to or less than 2.5 nm (above-described
preferred implementation mode 7). Preferably, the maximum height of
the magnetic layer, R.sub.max, is equal to or less than 0.5 .mu.m;
the ten-point average roughness R.sub.z is equal to or less than
0.3 .mu.m; the center surface peak height R.sub.p is equal to or
less than 0.3 .mu.m, the center surface valley depth R.sub.v is
equal to or less than 0.3 .mu.m, the center surface area percentage
S.sub.r ranges from 20 to equal to or less than 80 percent, and the
average wavelength .lambda.a ranges from 5 to equal to or less than
300 .mu.m. The number of surface protrusions in the magnetic layer
measuring from 0.01 .mu.m to 1 .mu.m in size is preferably set to
within a range of from 0 to 2,000, thereby electromagnetic
characteristics and the coefficient of friction are preferably
optimized. These can be easily controlled by varying surface
properties with fillers in the support, as well as by the particle
diameter and quantity of powders added to the magnetic layer, the
surface shape of the rollers employed in calendaring and the like.
Curling is preferably within .+-.3 mm.
When there is both a nonmagnetic layer and a magnetic layer in the
magnetic recording medium according to the present invention, it
will be readily understood that the physical characteristics of the
nonmagnetic layer and the magnetic layer can be varied based on the
objective. For example, the magnetic layer can be imparted with a
high modulus of elasticity to improve running durability while at
the same time imparting to the nonmagnetic layer a lower modulus of
elasticity than that of the magnetic layer to improve head contact
with the magnetic recording medium.
Embodiments
Specific embodiments of the present invention are described below;
however, the present invention is not limited thereto. Unless
specifically stated otherwise, "parts" refer to "mass parts".
TABLE 1 Type of Carbon Black Backcoating Layer Compound
Microgranular Coarse Granular Inorganic Amount Added Relative to
Carbon Black Carbon Black Powder Type Carbon Black (wt %)
Embodiment 1 MONARCH 800 Thermax MT None Compound 2 4 Embodiment 2
MONARCH 800 Thermax MT None Compound 2 10 Embodiment 3 MONARCH 800
Thermax MT None Compound 9 4 Embodiment 4 MONARCH 800 None None
Compound 2 4 Embodiment 5 MONARCH 800 Thermax MT Hematite Compound
2 4 Embodiment 6 MONARCH 800 Thermax MT Alumina Compound 2 4
Embodiment 7 RAVEN 2500 ULTRA Thermax MT None Compound 2 4
Embodiment 8 RAVEN 2500 ULTRA Thermax MT Alumina Compound 2 4 Comp.
Ex. 1 MONARCH 800 Thermax MT None None -- Comp. Ex. 2 MONARCH 800
Thermax MT None Phthalocyanine 4 compound (1)
The physical characteristics of the carbon black employed were as
follows.
(1) MONARCH 800 (from Cabot Cooporation):
Mean particle diameter 17 nm Specific surface area 210 m.sup.2 /g
DBP oil absorption capacity 68 cc/100 g pH 9.0 Volatile content 1.5
weight percent
(2) RAVEN 2500 ULTRA (from Columbia Carbon Co., Ltd.)
Mean particle diameter 13 nm Specific surface area 270 m.sup.2 /g
DBP oil absorption capacity 65 cc/100 g pH 5.7 Volatile content 1.2
weight percent
(3) Thermax MT:
Mean particle diameter 270 nm Specific surface area 8-11 m.sup.2 /g
DBP oil absorption capacity 30-40 cc/100 g pH 9.0-11.0 Volatile
content 0.3 weight percent or less
Embodiment 1
Magnetic Coating Material 1 Ferromagnetic metal powder 100 parts
Co/Fe=30 atomic %, Al/Fe=8 atomic %, Y/Fe=6 atomic % Hc:
1.87.times.10.sup.5 A/m (2,350 Oe) Specific surface area: 55
m.sup.2 g .sigma.s: 140 A.multidot.m.sup.2 /kg (140 emu/g)
Crystalline size: 140 .ANG. Major axis length: 0.068 .mu.m Acicular
ratio: 6 Surface oxide film thickness: 25 .ANG.
Vinyl chloride polymer MR 110 12 parts (from Nippon Zeon Co., Ltd.)
Polyurethane resin A 4 parts .alpha.-Alumina (mean particle
diameter 0.15 .mu.m) 5 parts Carbon black (mean particle diameter
40 nm) 5 parts Phenylphosphonic acid 3 parts Butyl stearate 5 parts
Stearic acid 6 parts Methyl ethyl ketone 180 parts Cyclohexanone
180 parts
Nonmagnetic Coating Material Nonmagnetic powder .alpha.-Fe.sub.2
O.sub.3 80 parts Major axis length: 0.12 .mu.m Specific surface
area by BET method: 50 m.sup.2 /g pH: 9 Surface treatment agent:
Alumina compound (1 weight percent as Al.sub.2 O.sub.3)
.alpha.-Alumina (mean particle diameter 0.15 .mu.m) 5 parts Carbon
black (Mitsubishi Chemical Corporation, 20 parts #950, mean primary
particle diameter 16 nm) Vinyl chloride polymer MR110 (from Nippon
Zeon 12 parts Co., Ltd.) Polyurethane resin A 5 parts
Phenylphosphonic acid 2 parts Butyl stearate 6 parts Stearic acid 5
parts Methyl ethyl ketone/cyclohexanone (7.3 mixed 250 parts
solution
The above-listed polyurethane resin A was synthesized as
follows.
A reflux condenser and a stirrer were procured; bisphenol A
hydride, a propylene oxide adduct of bisphenol A (molecular weight
700), polypropylene glycol (molecular weight 400), and
bis(2-hydroxyethyl)sulfoisophthalate sodium salt were added in a
molar ratio of 24:14:10:2 to a 50:50 mass ratio of cyclohexanone
and dimethyl acetamide in a vessel that had been backfilled ahead
of time with nitrogen; and the compounds were dissolved at
60.degree. C. under a nitrogen gas flow. Di-n-dibutyltin dilaurate
was added as a catalyst in a proportion of 60 ppm with respect to
the total quantity of starting materials employed. MDI
(4,4'-diphenylmethanediisocyanate) was added in a quantity
equimolar with the total sum of diol, the mixture was reacted with
heating for 6 hr at 90.degree. C., and polyurethane resin A
comprising 4.0 mmol/g of ether groups, incorporating
8.times.10.sup.-5 equivalent/g of SO.sub.3 Na, with an Mw of 45,000
and an Mn of 25,000 was obtained.
Backcoating Layer Formation Coating Liquid Composition (1)
Microgranular carbon black powder 100 parts MONARCH 800 from Cabot
Corporation, mean particle diameter: 17 nm) Coarse granular powder
5 parts (Thermal Black from Cancarb Limited, mean particle
diameter: 270 nm) Alcohol-free nitrocellulose 157 parts (from Asahi
Kasei Corporation, Sernova BTH1/2) Polyurethane resin 26 parts
Polyisocyanate resin 26 parts Polyester resin 4 parts Backcoating
layer compound (see Table 1) 4 parts Methyl ethyl ketone 1,300
parts Toluene 700 parts
With regard to the above-mentioned magnetic layer coating material,
the pigment, polyvinyl chloride, phenylphosphonic acid, and 50
percent of each of the solvents in the formula were kneaded, the
polyurethane and the remaining components were added, and the
mixture was dispersed with a sand grinder. To the dispersion
obtained were added 14 parts of polyisocyanate and 30 parts of
cyclohexanone, and the mixture was passed through a filter having a
1 .mu.m mean pore diameter to prepare the respective magnetic layer
formation coating liquids.
With regard to the nonmagnetic layer coating material, the metal
oxides, carbon black, polyvinyl chloride, phenylphosphonic acid,
and 50 percent of each of the solvents in the formula were kneaded
in a kneader, the polyurethane resin and the remainder of each of
the solvents were added, and the mixture was dispersed in a sand
grinder to obtain the nonmagnetic layer dispersion.
The dispersion obtained was stirred with a disperser and dispersed
with a sand grinder. To the dispersion obtained, in the case of the
nonmagnetic layer coating liquid, 15 parts of polyisocyanate were
added, after which 30 parts of cyclohexanone were added. The
dispersion was then passed through a filter with a mean pore
diameter of 1 .mu.m to complete preparation of the nonmagnetic
layer formation coating liquid.
Method of Dispersing Carbon Black in the Backcoating Layer and
Method of Preparing Backcoating Layer Coating Liquid
With regard to the backcoating layer coating material, the full
quantity of microgranular carbon black, 95 percent of the
backcoating layer compound and of the Sernova BTH1/2 called for in
the formula, and 47.5 percent of each of the solvents called for in
the formula were admixed, stirred in a disperser, and dispersed
with zirconia beads (1 mm in diameter) in a sand grinder. Next, 38
percent of the polyurethane resin called for in the formula was
added, the mixture was stirred in a disperser, and the mixture was
dispersed for 2 hr with zirconia beads (1 mm in diameter) in a sand
grinder.
The full quantity of coarse granular carbon black, 5 percent of the
backcoating layer compounds and Sernova BTH1/2 called for in the
formula, and 2.5 percent of each of the solvents called for in the
formula were admixed, stirred in a disperser, and dispersed for 6
hr with zirconia beads (1 mm in diameter) in a sand grinder. Next,
4 percent of the polyurethane resin called for in the formula was
added, the mixture was stirred in a disperser, and the mixture was
dispersed for 2 hr with zirconia beads (1 mm in diameter) in a sand
grinder, yielding backcoating layer dispersion a.
The above-mentioned backcoating layer dispersion a was stirred in a
disperser, dispersed for 30 min in a sand grinder, the remainder of
the polyurethane resin, polyester resin, curing agent, and
individual solvents were added, and the mixture was stirred with a
disperser to obtain backcoating layer dispersion b.
The backcoating layer dispersion b obtained was stirred in a
disperser and the dispersion was passed through a filter with a 1
.mu.m mean pore size to complete preparation of the backcoating
layer forming coating liquid.
Simultaneous multilayer coating was conducted on an aramide base
having a thickness of 5.5 .mu.m and a center surface average
surface roughness of 2 nm by coating the nonmagnetic layer forming
coating liquid in a quantity yielding a dry thickness of the lower
layer of 1.7 .mu.m and immediately applying magnetic coating
material 1 thereover in a quantity yielding a magnetic layer 0.20
.mu.m in thickness. While the two layers were both still wet, they
were oriented with a cobalt samarium magnet having a magnetic force
of 4.8.times.10.sup.5 A/m (6,000 Oe) and a solenoid having a
magnetic force of 4.8.times.10.sup.5 A/m (6,000 Oe) and dried.
Subsequently, backcoating layer forming coating liquid (1) was
applied to a thickness of 0.4 .mu.m. A seven-stage calender
comprising only metal rollers was then used to heat treat the
product at a temperature of 95.degree. C. at rate of 150 m/min.
Next, slits 3.8 mm in width were formed in the resulting coated
product, the magnetic layer was subjected to a surface abrasion
treatment, and the coated product was wound into a DDS cartridge to
obtain a sample (magnetic tape). The magnetic characteristics of
the magnetic layer obtained and the Ra of the magnetic layer and
backcoating layer were measured. The 4.7 MHz reproduction output,
C/N ratio, coefficient of friction at 23.degree. C. and 60 percent,
guide pole grime, and head grime were measured.
Embodiment 2
With the exception that the quantity of backcoating layer compounds
added was changed to 10 wt percent, a magnetic tape was produced in
the same manner as in Embodiment 1.
Embodiment 3
With the exception that the backcoating layer compounds were
changed to the types shown in Table 1, a magnetic tape was produced
in the same manner as in Embodiment 1.
Embodiment 4
With the exception that no coarse granular carbon was added to the
backcoating layer, a magnetic tape was produced in the same manner
as in Embodiment 1.
Embodiments 5 and 6
With the exception that the types of inorganic powder indicated in
Table 1 were added and backcoating layer dispersion c below was
employed, a magnetic tape was prepared in the same manner as in
Embodiment 1.
The full quantity of inorganic powder (hematite or alumina), 5
percent of the polyurethane resin called for in the formula, and 10
percent of each of the solvents called for in the formula were
added, stirred in a disperser, and dispersed for 3 hr with zirconia
beads (1 mm in diameter) in a sand grinder.
Backcoating layer dispersion a was stirred in a disperser and
dispersed in a sand grinder for 30 min, after which the remainder
of the polyurethane resin, the polyester resin, the curing agent,
and the remainder of each of the solvents were stirred in a
disperser to obtain backcoating layer dispersion c.
The following inorganic powders were selected for use.
Hematite 15 parts (TF100 from Toda Kogyo Corp., mean particle
diameter: 110 nm, Mohs'hardness: 9) .alpha.-Alumina 15 parts
Embodiments 7 and 8
With the exception that the microgranular carbon black indicated in
Table 1 was employed in Embodiment 7, a magnetic tape was produced
in the same manner as in Embodiment 1. With the exception that the
microgranular carbon black shown in Table 1 was employed and
alumina was employed as the inorganic powder in Embodiment 8, a
magnetic tape was produced in the same manner as in Embodiment
1.
Comparative Example 1
With the exception that no backcoating layer compound was added, a
magnetic tape was produced in the same manner as in Embodiment
1.
Comparative Example 2
With the exception that the phthalocyanine compound (1) described
in Japanese Examined Patent Publication (KOKOKU) Heisei No. 7-31801
was employed as the backcoating compound, a magnetic tape was
produced in the same manner as in Embodiment 1. ##STR12##
Evaluation Methods
(1) Center surface average surface roughness (Ra):
Surface roughness as measured by optical interference roughness
meter (Ra): Using a TOPO3D from WYKO Co., Ra, Rams, Peak-Valley
value of a surface area of about 250.times.250 .mu.m were measured
with an optical interference roughness meter. Spherical surface
correction and cylindrical correction were subjected at a
measurement wavelength of about 650 nm. The optical interference
roughness meter employed was a non-contact surface roughness meter
that measured by optical interference.
(2) First pass coefficient of friction (.mu. value) and 500.sup.th
pass coefficient of friction (.mu. value) of backcoating layer
surface: The tape was passed at an angle of 180 degrees over an
SUS420J with a diameter of 4 mm, slid at a rate of 18 mm/sec and a
load of 10 g, and the friction of coefficient was calculated by
Euhler's equation.
The measurement was conducted through the 500.sup.th pass. The
coefficient of friction .mu.1 of the first pass and the coefficient
of friction .mu.500 of the 500.sup.th pass were calculated.
(3) Guide pole grime: The grime on the guide poles touching to the
back surface was evaluated after running the cartridge five times
on a DDS drive.
The grime on the guide poles was visually observed, wiped with
tissue, and functionally evaluated. The larger the number, the more
grime there was in a five-level evaluation. Five was the level with
the most grime on a scale of from 1 to 5.
TABLE 2 Backcoating Backcoating Layer Characteristics Layer Surface
Coefficients Rough- Surface of Friction ness Resistivity First
500.sup.th Guide Luster (nm) (.OMEGA./sq) pass pass Grime Embodi-
126 6.0 3 .times. 10.sup.5 0.25 0.27 1 ment 1 Embodi- 135 5.6 1
.times. 10.sup.5 0.25 0.26 1 ment 2 Embodi- 122 6.1 3 .times.
10.sup.5 0.25 0.26 1 ment 3 Embodi- 126 5.9 4 .times. 10.sup.5 0.25
0.26 1 ment 4 Embodi- 125 5.8 3 .times. 10.sup.5 0.25 0.26 2 ment 5
Embodi- 125 5.8 3 .times. 10.sup.5 0.24 0.26 1 ment 6 Embodi- 124
5.9 4 .times. 10.sup.5 0.24 0.25 1 ment 7 Embodi- 124 5.9 4 .times.
10.sup.5 0.26 0.27 1 ment 8 Comp. 5 Not 8 .times. 10.sup.4 0.25
0.28 4 Ex. 1 measur- able Comp. 90 13.3 3 .times. 10.sup.5 0.25
0.28 3 Ex. 2
The results of Table 2 reveal that relative to the comparative
examples, the embodiments of the present invention had lower
surface roughness (Ra) of the backcoat layer, had greater luster,
were smoother, and had less grime on the guide poles. The initial
running coefficients of friction were similar and the change in the
coefficient of friction with repeated running was stable. This was
attributed to the fact that the backcoat layer was applied as a
carbon black coating material obtained by dispersing a mixture
comprised of carbon black having a mean primary particle diameter
of from 5 to 30 nm, the compound denoted by general formula (I)
above, and binder, thus increasing the affinity of the compound of
general formula (I) (the derivative portion) with the binder and
the carbon black (surface acidic group), increasing dispersibility,
preventing dropout of particles such as carbon black, making the
layer smooth, and reducing guide pole grime.
In Comparative Example 1, the compound of general formula (I) was
not added to the backcoating layer. Compared to the embodiments,
luster and surface roughness were poor, guide pole grime was
marked, and the coefficient of friction with repeated running was
high.
In Comparative Example 2, phthalocyanine compound (1) was added in
place of the compound of general formula (I). Compared to the
embodiments, luster and surface roughness were poor, the
coefficient of friction with repeated running was less stable, and
guide pole grime was marked.
Since the magnetic recording medium of the present invention is
smooth, generates little grime, and has dropout due to particle
fallout and the like, it is suitable not just for use with
conventional inductive heads, but also with MR heads in which noise
is critical.
The present disclosure relates to the subject matter contained in
Japanese Patent Application No. 2000-299711 filed on Sep. 29, 2000,
which is expressly incorporated herein by reference in its
entirety.
* * * * *